Not that I am a big believer in salt in the water there are times when this can be benificial. Salt contact with the bare aluminum would accelerate the corrosion since there is a chemical reaction between the two.
One product that is being used often to seal concrete ponds is Sani-tred. While not knowing about it's specific ability to seal to aluminum, you could call the company and ask. It seems to adhere to many different surfaces so can't see why it wouldn't work:
http://www.sanitred.com/WaterFountain.htm
Rick