oooh, where are you going?
I'm British but I've visited a fair few aquariums in the US on various trips!
If you are just staying in NY the NY Aquarium in Coney Island is a good visit, take the metro down to the Island and if the weather is nice then you can go on teh fun fair too!
If you are traveling further afield there are aquariums in Washington, Boston and Chicago that are brill, although the so-called National Aquarium in Washington is a bit of a wash out, they have a few good tanks!
The Boston Aquarium is worth the trip if you can make it, they have a MASSIVE tank that you can walk around and up and down, with all kinds of things in it. Quite impressive.
As you can see, I've only been to the ones in the touristy places, I am sure some residents can let you know of smaller ones that are off the beaten track.