Anticipating what you will find when you do get it off --
These crappy pumps -- and I have one -- are made of poorly finished die cast aluminum
. The valve seats corrode and fail to seal if any sea water
is left standing in them, rendering the pump nonfunctional.
You will have to sand or grind the valve seats smooth, then finish them somehow -- I used zinc chromate primer and Hammerite.
Then be sure to never leave sea water
Better yet, replace it with something made of material which does not deteriorate in sea water.