Water shoes are a good idea. That way, you can come and go as you please and not have to worry about hot sidewalks, rocks, etc. Also, when you're snorkling, all the coral is fake so you can step on it and water shoes would be good there, in my opinion.
You can get cheap ones at wal-mart, target, etc. They may be easy to find now with end of season, or hard if your store has already changed to the fall season stuff.
You can find better ones online at different athletic stores.
They have some in the gift shop, but from what I remember, they were over $20 a pair.
HTH,