Swimmers itch is usually from jellyfish larva, can be an issue in FL and the carib certain times of the year but sunscreen usually helps protect you, just remember to put it all over, not just where the sun shines! Your LDS also sells a protection cream.
Some places like the Chesapeak Bay have a bigger jellyfish problem and I got nailed on the lips doing a giantstride into a NJ inlet once, it seemed like the top 10" of water were solid jellyfish.
Real parasites are out there, a freind died of a parasite he picked up in Egypt but that may have been in not so fresh water during a baptisim.