Did you figure out the problem? I mean did this pump problem repeat once again? Cause I think I've got the same one with my pond aerator. I know that it is different from that one on your boat, but the situation is alike to that one you described.
I've ordered that stuff from amazon, and you know lucky you that you've canceled it. Just a waste of money. I tried the recipe that you shared above, and that tastes much better.