Vs. Snake has recently become one of my favorite MUs. It's so easy.
First and foremost, you must NOT FEAR GRENADES. If you play super cautious because you are worried about grenades, you will lose to good Snakes, because you will miss far too many opportunities. Having a good timing of three seconds so you know when grenades will go off is good. I hate watching two players shield for an entire 50 - 120 frames in fear of a grenade.
Now, Fair is your primary zoning tool. However, Snake is a character you should aim to hit, not zone. You beat out ftilt, but if you mess up you're spacing, you're going to get hit. When Snake pulls a grenade, dtilt is actually a fabulous option, because while Snake is holding the grenade in his hand, dtilt will not set it off. Good spacing and proper move selection will always prevent you from setting off grenades. For this reason, Dancing Blade is not as good in this MU as you might imagine, though it still has amazing useful utility for juggles.
Now, when grabbing Snake, if he's at higher percents, send him into the air. However, at lower and mid percents fthrow is a godsend, because Snake really doesn't have ANY options, and Dancing Blade is virtually guarenteed as a follow up, even if Snake airdodges. His only way to evade it is to jump (LOL) and I have not yet experimented with SHDB to remove all of Snake's options. I've yet to have a Snake pull out a grenade on my after an fthrow, but if they do, just regrab.
Now, a lot of people have difficulty getting Snake into the air. After all, Snake isn't just going to let you grab him, or get a Uair/Utilt, and grenades make Dancing Blade tricky. For this reason, I find extreme success in pressuring Snake to the edge. It's very easy to pressure Snake once you land a single hit, because his options get cut every time you hit him. At 0 out of grab, I fthrow to Dancing Blade (though you can probably fthrow to Nair to Dancing Blade, I just like to keep my stuff on Snake SUPER safe). Remember that since Snake is a heavy weight character, your throws will have slightly more lag on the end of them (odd but sensible feature bites us here).
Once Snake is on the edge, he really doesn't have any reliable way of getting back onto the stage without getting hit. Remember that Snake has 0 landing lag if he's holding a grenade when he lands, so if you're going to punish with dtilt, make sure you swing slightly early to still catch him while he's in the air. Dtilt - Dancing Blade is staple for juggling and pressuring Snake back to the edge.
Now, when you hit Snake offstage, many Snake's like to jump and UpB immediately to escape high. This is your opportunity to Fair Snake, so he doesn't get the bonus of rising super fast. Dairing him while he's not close will be easy. If Snake goes low and tries to C4 himself, footstool.
Now, smart Snakes will recover by going far out, C4ing themselves out of your range, and then UpBing again far above the stage. This is obviously where you set up your juggle. Jump as to Uair Snake. If they pull a grenade, you should space the Uair to tip Snake, and it should not set off the grenade. Be cautious of FF Bairs, and wait for airdodges, but not too long.
Obviously, standard punishing applies if Snake tries to airdodge to the ground in front of you. Expect Snake to live to high percentages, and don't worry about staling your smash attack, be more concerned with racking up damage. If Snake's at 140%, you can refresh half your moveset with a grab and pummeling, and rack up a lot of damage, while putting Snake in a bad place. This is your kill set up. Usmash doesn't kill until over 140% anyway, so don't worry about staling it.
Unless Snake reads a roll (and rolling is not generally a good idea vs. Snake, Fair is almost always better unless you need to evade a grenade or something) or just does something amazing, you really shouldn't get hit by Utilt. When you hit around 95% (you should ban Halberd), then you know Snake will be looking to use this move. There isn't really a good reason to get hit by it.
Your worst enemy in this MU is actually Dash Attack. If you try to space Snake instead of simply hitting him, he can easily break zoning with a DA. It's much like Phantasm in this sense, for those of you with Falco experience, but it's actually much easier to punish. Snake players are away that Marth's Fair wrecks them, and it's easy to pick up on DA patterns. If you want a safe way to punish it without leaving yourself open if you guess wrong, simply jump backwards and FFNair. Jab is also an okay option.
Another huge reason why Dancing Blade isn't great in this MU is because it's transcendent. Normally, this is a life-saver for us, because it has great range and we ignore other characters priority with it's blinding speed. However, in this MU, Snake has a more powerful move with equal speed and range (ftilt). If Dancing Blade and Ftilt trade, you will be at disadvantage, because Snake's ftilt has more hitstun than DB1. If you use dtilt or jab and trade, either jab repeatedly to block additional ftilt attempts, or just shield. UpB is too risky in case your opponent shields. Either way, Dancing Blade should ONLY be used to punish during trapping, or finish a combo.
Dtilt is an excellent move. As I said before, you can use it to prevent Snake from landing with good timing, and if Snake pulls a nade on the ground, it's still a safe poke (though a Fair does good as well.) Shield breaker is amazing for obvious reasons.
If you jump out after Snake, and he manages to get away on the cypher, using Breversal shield breaker can help you get back to the stage faster.
If Snake is Cyphering and he's out of range of a Dair but still in range for Fair, you can UpB in a similar fashion to MK. You'll hit the cypher (make sure to DI it appropriately). It can potentially kill, but is just good for dealing more damage mostly. If you don't use your second jump, it can actually set up for a spike if you properly DI the cypher.
Two super important tricks to fighting Snake are these:
A) Pivot grab his fall to the ground at the front of him. Snake can't do anything. Bair hit's behind him, and this pretty much catches all his options. Then Uthrow.
B) If Snake drops low, and then jumps, just grab the edge. Pay attention to the way Snake's like to grab the edge. Snake's don't want to use UpB, so they'll grab the edge when they can. Hogging a jump to the edge is an easy spike after, so don't try and dtilt them, because they're good at spacing a jump to the ledge. Just hog, and then if they go up, punish with Uair from the ledge.
I think that Snake vs. Marth is probably even right now, and my opinion on it totters either way depending on the day. At Snes I MMed Ally, and I won game one, but he beat me game two and two stocked me game three. I should've beaten Jesiah IMO, but I SDed (johns, so what). Other than that, I've pretty much been ****** all the Snakes I've played, (I haven't fought Fatal in singles yet but he ***** me at in doubles with ADHD, I beat Razer in friendlies at Genesis in Singles and Doubles.)
Just make the right reads, and play a smart safe game, without fearing Grenades too much. Oh, don't pick them up. If you do, shield unless you have ample time to throw them away.