Snake
First and foremost—leave the grenades alone. Don't pick them up, don't try z-throwing them, and definitely don't glide-toss. They're Snake's, and he has way too much control over them, even when they're in your hands.
At low percents, learn to abuse f-throw -> db—it's a godsend—don't forget to buffer the dash. Due to Snake's weight, this continues to work past the normal 0-3% f-throw chains that Marth has on most characters. Abuse it, learn it and use it for early leads.
Once Snake reaches low-mid percents, the knockback from f-throw will keep dancing blade from linking; at this point, Snake has limited options, all of which Marth can cover. If he airdodges into the ground, beat it out with a d-tilt—this will also cover him pulling out a grenade, due to the safe hitboxes being his feet and his head. If he jumps toward you, greet him with an u-air. If he leaps away, a carefully placed f-air will put him off-stage without a jump, setting up a cypher-spike.
A few final notes on dancing blade in this match-up:
—a shielded dancing blade always means a ~20% f-tilt for Snake.
—Snake can escape our down/green finisher with grenades.
—if you shield a f-tilt, a buffered dash out of shield allows us to punish Snake's lag with a dancing blade.
The goal of the above is to gradually push Snake toward the edge. While you shouldn't constantly be fishing for them, ledge-setups can come from early strings. If you do happen to get him near the ledge, Snake doesn't have very many options outside of grenades and rolls—just be wary of unexpected grabs/f-tilts—if you're smart, you should be able to push him off-stage.
As far as edge-game goes, I honestly believe the following to be one of Marth's best options: b-throw.
If you're back is to the ledge, and you b-throw your opponent off the stage, what can Snake do?
—Fall to the ledge? You speed hug, or w/e it's called, forcing them into a gimpable situation; whether it's a ledge-dropped fair -> up-b -> stage-spike, or a d-air, they're now below the lip of the stage, and no one ever wants to be there against Marth.
—Double jump back onto the stage? Use that beautiful b-air that tips o-so-easily, regrab or simply wait for that airdodge -> d-tilt -> cypher spike.
—Up-b? LOL... poor Snake.
I'm not going to preach about Snake being off-stage equaling an easy ~25% or death, because if you play the match-up based solely off of that philosophy, you won't win.
However, this is Snake at his most vulnerable, so capitalizing on it should become second-nature. There are two ways for Snake to come back to the stage: from high or from low.
If the Snake choose to ride his cypher all the way up to the stars, then it's juggle time. Do not let the phrase "juggle" limit you to u-airs. While they're great, and when tippered, manage to avoid Snake's grenades, they aren't the only way of keeping your opponent in the air. Being creative here is must, because from my experience, a double-jumped, fresh u-air can kill Snake off the top any time after ~150%—DI-dependant, of course.
With this in mind, there are several ways to reset this situation, while keeping this very viable kill-move fresh:
Snake has three ways of coming back down to the stage when recovering high: with a grenade out (he may pivot this), airdodging into the ground or throwing out an aerial.
I'm not going to cover every single option we have here—by now, most of them should be self explanatory. However, I would like to point out, that a u-tilt from beneath Snake can outspace all of his falling-aerials and his grenade hitboxes. For me, this move is usually fresh, which means, tippered, can kill early. Careful of the airdodge, though—u-tilt has plenty of ending-lag on it for Snake to punish if whiffed.
Likewise, there's always grabs. These will always lose to falling aerials, but beat out any grenade tricks (don't pummel, unless you've the higher port). Combine this with the b-throw tactic mentioned above, and you've set Snake up for even more punishment.
Finally, if he's falling near the ledge, or even the center of the stage at higher percents, a tippered d-tilt will beat out his airdodges, grenades and aerials, while sending him off-stage at a low angle, leaving him open for more cypher spikes.
On the topic of low angles, if you manage to force Snake to cypher from below, they can either ride it to the ledge, or airdodge off of it into the stage.
If it looks like they're going for the ledge, simply speed-hug. Guessed wrong? Don't fret, n-air him as he comes out of his airdodge/pulls out a grenade/b-airs—tipper these, not just to stay safe from grenade explosions, but this could result in resetting the situation on the other side of the stage or even netting you an early kill. If they ride it above, be ready for the airdodge—punish as mentioned above: grab & d-tilt both put him off-stage, resetting the situation.
I cannot stress enough, how good d-tilt is in this match-up. Not because it's quick, has IASA frames or is safe on hit, but because it absolutely destroys airdodges into the stage; which is something Snake just can't afford to take.
Unfortunately, Snake isn't always in the air.
When dealing with a turtling Snake, never approach with a dash. Nine times out of ten, Snake will be able to get a f-tilt off before our shield comes up. Most of your approaches during this match-up should involving walking.
As mentioned before, it is safest to hit Snake at his feet and head when he has grenades out—d-tilt works extremely well for the former, but should not be used when Snake is in shield, only when he is falling. The reason for this is, of course, shield-dropped f-tilt, which Snake can't do in the air.
Also note that tippered aerials can also be used as an approach, since they circumvent Snake's grenade hitboxes, as well. Your best choice for this category would be a rising f-air, as both a falling f-air and n-air will, once again, result in eating a f-tilt due to lack of shield-push from Marth and a massive f-tilt range from Snake. Double-jump away if you whiff the f-air to reset the situation.
F-tilt, f-tilt, f-tilt:
Before we get into punishing/responding to f-tilt, it must be understood that this move should rarely kill you. If you get into the habit of DI'ing during the first hit, then this move won't kill you until much later into your stocks. In my opinion, learning to DI the f-tilt is just as important as avoiding the u-tilt, when it comes to surviving against Snake.
For starters, simply shield the f-tilt and punish with dancing blade, as mentioned above. This alone, should keep you from attempting both grabs and dolphin slashes—shield push is nasty, and neither of these have the range to reach Snake.
If Snake is using the first hit of f-tilt to poke, in the same manner that we would use DB1 to poke at a shielded opponent, or if he is especially fond of jab -> f-tilt, a dolphin slash OOS will stop him in his tracks.
Just as with all match-ups, be one-hundred percent wary of using dolphin slash. It is neither a completely offensive or defensive move—it is, however, completely situational and should only be used if you are certain it is going to connect. There are no guesses/reads worth risking a whiffed dolphin slash, especially against a hard-hitter such as Snake.
As a last remark on dolphin slash, a Snake with even remotely decent DI, will live this move even at the most extreme percents, i.e. ~160-180%, another reason why high-rish/high-reward just doesn't cut it in this match-up.
On the note of killing Snake:
I find that, more-so in this match-up than others, I end up perfect-shielding a lot of Snake's boxing moves—not intentionally, but it happens nonetheless. If you perfect-shield anything of Snake's, respond with a shield-dropped d-smash instead of dolphin slash. As mentioned above, dolphin slash doesn't kill a Snake with good DI, even near the edge of the stage, until later percents. This, combined with the fact the d-smash kills vertically and tips incredibly easy, makes it far more efficient at KO'ing Snake.
A few side notes:
If you find that you're being destroyed by n-air, it's probably because you're "returning to the ****." This is simply the desire to get back to your opponent as quickly as possible to hit them with whatever move you think will connect. I'm sure we're all familiar with this feeling, the one where you're constantly DI'ing toward that Snake for that f-air, even though you've been sent off-stage twice before... by the exact same move.
Just take a moment to breathe, get to the ledge. You're Marth, you don't have to have your feet on the ground at all times. Plank a little, get your head together.
Also take note of how the Snake DI's during a n-air. If they're fading backward, away from you, it is not safe to dolphin slash. If they're DI'ing forward, or not at all, then start the button mashing. Don't want any freefalls -> f-smashes against Snake.
There is no "set-in-stone" way to deal with a Snake that loves d-throw; you simply have to be unpredictable. I tend to stay away from rolling behind Snake, simply because it is usually a guaranteed f-tilt for him, unless he incorrectly reads a forward roll.
From a shielded position, Snake can shield-drop f-tilt to punish a backward roll and also regrab both a stand-up and get-up attack, putting you back into a d-throw and resetting the situation.
I'm not saying roll forward every time, because that's just as bad as the above. Pay attention to what the Snake does when he lays you down, because a d-throw, with even one pummel in between is an easy ~15%—something that can't be afforded when we die to u-tilt as early as we do.
When approaching Snake with a grenade out, be alert for an unexpected dash attack or grenade-dropped grab; the latter leads to a d-throw -> grenade explosion unless you have the higher port.
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If you take anything from this, let it be d-tilting airdodges, b-throwing near the ledge and learning to read your opponent after a f-throw early game—all three of these are universal.
Beyond anything you read that I write; you have to learn the match-up. Spending time playing match-ups is the only true way to learn them. During practice is when it changes from reaction to instinct, instinct to second-nature and second-nature to a completely elevated light—it's something that can't be read about, because it can't be put into words. No one, no matter how good they are, can explain to you how to be good, as well.
