You cannot swim. There is bacteria in the water and I've heard there is the occasional alligator as well.
I think anyone can walk or play on the beach, you do not have to be a resort guest.
In march of 07 we stayed at the Poly and there were TONS of cigarette buts in the sand and my toddler kept trying to eat them. And I mean like every bucket of sand there were maybe 6 cigarette butts. So I don't really recommend letting very young children play in the sand. It was unbelievably disgusting. They do clean the beach every day with a rake, but not something that could pick up cigarette buts. I guess beaches look like ashtrays to way more people than I ever imagined. It is really too bad that smokers had to ruin the beach.