I've just started seeing these little guys in freshwater here on Lake Martin in Alabama. In the summer, we come to our place here to get away from high heat and hurricanes in Playa del Carmen. The weather is so nice and the water is perfect temperature. These jellies are found near the surface in the morning and later in the afternoon. They are about the size of a quarter, tiny and to my knowledge, harmless.
We put together the music in Garageband and edited down a little over 1 minute worth of footage outlining the adventure of the jellyfish hunt. No jellies were harmed in the making of this movie :)