Aluminum trailer goop ?
 

Whats the best stuff to keep a bare aluminum trailer protected in the salt ?

A hose.

Yep.

Aluminum and salt aren't very compatible. Dunking an aluminum trailer into salt water is going to lead to corrosion regardless of what steps you attempt. The worst thing is that your hose isn't going to get into the places that dunking it does. You'll have a long battle that you'll eventually either lose or give up on.