If they approach via wavedash in, dtilt, Dancing Blade, jab, and fair (if you're in the air) are all very fast, and dtilt can avoid bair (not to mention, Dancing Blade stops you instantly). Nair and dair drifting away are both fairly useful when they're uncomfortably close. Dash dance is great, especially if they shield. Try to just stay out of bair's range, and be ready to punish any desync or WD forward attempts. If ICs are nearby, but don't have the opportunity to shield utilt is strong. Try to do fewer attacks, and more movement, it's easier to not be the proactive one. Stay close and watch for any desync attempts though, you don't want to deal with blizzard walls or ice block spam. Try to space gabs with shield stops if you go for them so that you drag a climber in to make the other punishing you harder.
Ftilt will clank with Iceblock, reducing its already low lag (and it's Marth's longest ranged move on ground). This doesn't mean you can't be punished, but it's something to consider since you can avoid going to the air with this option.
If you like going to the air a bit more, SH facing away dair or bair mixup can be hard to come in on and counter CC.
Again, I recommend dashing after most aerials. Low fair to dtilt is also fine when both will tip (and when the dtilt will punish some CC or desync attempt).