Lots of good advice. If you are shooting over decoys, I would say you could even open up to Imp Cyl. I only use 2-3/4" shells, #3 is ideal for mallard size, go to #4 for teal. But look for at least 1500 fps, rather than a heavier payload. Take TWO boxes with you, not just one. Nothing worse than running out of ammo during a hunt.
But more importantly than your original question, is are you a capable wingshooter? If this is all new to you, spend a couple of sessions at the local skeet field. Do at least four rounds of skeet to build some experience. On the marsh is an awful place to 'shoot all the misses out of your gun' (you know that saying, right?). Rather than having a frustrating day in the blind with two empty shell boxes and no birds, you'll have greater enjoyment and success.
