I don't think swimming in the ocean will allow very much absorption of salts, and therefore nothing to be overly concerned about, but I would be more concerned about the pollution that may also be absorbed thru the skin as a result.
Warm to hot water baths are known within Natural Hygiene circles to be extremely good for alleviating aches and pains, but an Epsom Salt bath will allow much absorption of magnesium thru the skin, which is beneficial nutritionally.
I am not of the opinion, one way or the other, whether that is a good practice while fasting or not.