A simple solution to providing a shower if you are skiing etc and want to wash off the salt water if you are staying out on the boat for the day.
http://www.mountainwarehouse.com/Health_and_Hygiene/Toiletries/sc1450/p16572.aspx?utm_source=google&utm_medium=GoogleSho pping&utm_campaign=googlebase&utm_term={keyword}
or
http://www.overstock.com/Sports-Toys...tml?cid=133635
Basically camping showers. Its a plastic bag you fill with water that has a tube and shower head on the bottom of it. The bag will warm in the sun but could be put in a jockey seat then pulled out and hung on the A frame as it is gravity fed.

May or may not be the solution you are looking for but cheap. £8