how to flush out gas tank thats in a boat hull?

Answer 1

I had to clean mine on a '89 javelin.
I syphoned out all I could, then removed the filler hose from the tank . I fashioned a swab out of an old T shirt & a stiff piece of wire & swabbed the inside out. Carberator cleaner works good for cutting thru any gunk you run into. My tank was sorta semi-see thru.
I did this while it was on a trailer.
Good luck

Answer 2

Is you can get the sender unit out this is the best way .go in an pump out with a non electric pump gravety feed from rear of tank .lift boat high if front. Be safe.