Forget all those remedies. Use a little prophylaxis against sea nettles.
 Here's the secret....
 Coat all your exposed parts with a heavy layer of petroleum jelly!

 Of course, there's the hazard of  falling on the sand when you are coated
that way.
  If that happens, you'll need to be sprayed down with Foamy Gunk or run
through a brushless car wash to get it off.

  Actually, some folks say that a light layer will do the trick, however
I'd prefer a sting to the degreasing job afterwards.

  George