Here:
Let me give you a layout of how hard it is for marth to approach.
1: Marth jumps at Samus with his standard fair approach. Samus can just jump backwards and zair and if the tip hits marth will be knocked back and if any other part hits marth will be stunned long enough for samus to just jump again preparing for another zair.
2. Marth can counter. If you just move backwards when he counters your zair its a free dash attack or grab.
3. Marth can airdodge. This is the hardest to counter. But Samus can. Samus needs to zair to force the airdodge, then grapple immediatly when she hits the ground and she will grab marth right when he lands.
4. Marth cannot approach with full hops because samus's uair and up B are amazing and marth is weak from below.
5. Samus's jab reset owns marth. She can jab him before his fair comes out, then ftilt or fsmash.
6. Samus can combo marth better then marth can combo samus.
7. Samus can double homing missile and marth can't just swipe them away.
8. Samus's Dtilt outranges all of marths moves.
9. Samus can gimp marths recovery with nair/dair/ homing missile to run off grapple. If he doesnt have his second jump and you zair him he dies.
So basically, on stages where you can keep marth a grapple distance away, you will own them. If they do manage to get inside your grapple range, remember the amazing shield parry in this game. That mixed with samus's jab game makes it hard to do things to her without being interupted. DO NOT LET MARTH FAIR YOU. You will be destroyed if you let the marth fair you alot. Also, stay on the stage aside from edgeguarding. Marth is also very hard to kill with samus. I reccommend using zair to chargeshot then edgeguard. Also, dtilt kills now.
On smaller stages where you cannot zair easily, marth destroys samus because of his fair combos and his ability to keep her above him.
Also, Remember that the zair has 0 lag.
I have tested all of this out and played tons of marth vs samus.
Edit: Watch this video:
http://www.youtube.com/watch?v=9zxz8irL2e4
It is one of my older videos from last week so I hadent realized how amazing zair is, but when I do use it notice how much it stuns and how amazing it is. Best move in this game.