Tiny water bugs and jellies in Riis Park?

I went to Jacob Riis Park (the beach) on Long Island yesterday afternoon, and after half an hour in the water I noticed these gross, tiny, 1/8th inch long little black bugs. I shrieked, and ran out of the water, seriously. Also, in the water I saw multiple small jellyfish-like organisms that did not sting. What are both of these things, and are they in all beaches? Have I just not noticed weird bugs in other beaches before? Is Riis just a favorite of icky small creatures? Help! I love Riis Park otherwise, but those bugs were seriously disgusting.