Aluminium to stainless in a salt water environment is a very very very bad idea.
Aluminum and stainless steel isn't too bad. They both form inert layers on the surface (which is why they don't corrode in general). So even though they have galvanic potential, they mostly insulate themselves.
I'm afraid to say Dave that you not entirely correct. Do aluminium and stainless steel have passive oxide layers? Yes they do. Does that mean they will not corrode when placed together? Not exactly.
Corrosion is an electrochemical process. It relies on voltage potential difference and a suitable electrolite solution.
For harsh enviroments (salt water) you want less than .15v difference in potential. The difference between aluminium and stainless is a minimum of .25v difference.
So the potential for corrosion is high. The next thing that needs to be considered is temperature. The higher the temperature the faster the corrosion. The temp in the sea isn't too bad. But then imagine placing a wet salty board in a hot car. Temperatures can be 40+ Deg C. That will dramatically increase the corrosion potential.
The next thing that increases corrosion speed is a crevice. The handle/bolt joint is a perfect crevice. The salt water gets in. Then the water evaporates leaving the salts. The next session brings more salt water which increases the salt concentration. The increased salt concentration increases the corrosion rate............ At temperature and you have a recipe for high corrosion.
Now in normal enviroments you can get away with .25v potential difference and in controlled conditions up to .5v difference.....
So to stop or slow corrosion insulate as masisback states. Also washing down with fresh water after every session and remove the salts.
A classic example of general lack of understanding in a given practical application.
Thats the problem with no experience.
Lots of it about....
I am still imagining putting a wet salty board in a hot car..the horror...the horror...