Blisters After Contact With Sea/ Fresh Water

I am a 39 year old male and I have never had a problem going into the ocean or a fresh water lake.
Recently I went out into the ocean for about 4 hours. Within an hour my hands started itching badly and tiny blisters started forming around the base of my thumbs and on the top of my hands. The blisters look like tapioca and are realy small. There are a lot of them and the fluid inside looks yellowish. The initial bad itch disapears after about a day or two although scalding hot water makes the itch go away immidiately for a good two to three hours. After about a week the blisters disappear and the skin turns brown and itches. A few days after that the dead brown skin is gone and the underlaying skin is red. It looks like recovering skin after a burn. The red skin is sensative to the touch but reduces in color to normal skin color after about another week.
The same thing happened in a freshwater river. The blisters came back in full force and almost dubbled in amount and spread to the bases of my index and middle fingers.
I went to see a dermatologist and he gave me "desoximetasone". It does help with the itch and the redness of the skin but it does not cure the problem. He said that there was no cure for this and just had to live with it. I don't have these symptoms with normal tab water.

Does anybody know about this condition and is it true there is nothing for it? Clearely there is something in "outdoor" water that causes this to happen. I hate having to give up my outdoor water activeties.

Thanks.