There is an interesting article in this weeks San Pedro Sun http://www.sanpedrosun.net/04-133.html
It talks about there being different forms of pica pica. People will react differently depending on what type they encounter and also on their individual sensitivity to the allergic reaction. It also talks about how you will not always be able to see the pica pica and that the brown thimble jellyfish that you see this time of year is not what actually gives you the pica pica.
It should also be mentioned that prevention is best accomplished by not wearing unnecessary items like wet suits or tee shirts. One normally gets the pica pica when it becomes trapped between your skin and outer items like a wet suit. It will normally not sting you if it can slide past you and not get trapped. As was mentioned in a previous post, the rashes will show up where clothing has allowed it to become trapped, like under your wet suit or swim trunks.
It also helps to apply oil to these areas to facilitate the pica pica to slide past. That is why many of the local commercial fisherman swimming for conch this time of year use vasoline. I have not tried the SafeSea, but have heard good reports about it. I use a local product from Mayan Secrets, mayansecrets.com, the cahune oil as a preventative and their Rain Forest Refresher for when I get it to help with the itching. Now last year when I talked about using oil, some of the people expressed their objection to using an oil. This oil is from a plant and not a petroleum product. But if some of you still object, that is fine. Enjoy your itching while thinking you are saving the planet.
It also helps to remove everything and rinse off as soon as possible. I carry fresh water on my boat for this purpose and require all attractive female divers to do this as soon as they exit the water.
If you take all these precautions and still get it, try taking antihistamines and applying the different remedies mentioned.