2012 Mercury 25 EFI breaks down under load (only in water)

[battle]

Hello and thanks for any help you can provide in advance. My 25 EFI runs great sitting in the yard (hooked to the hose of course) but once in the water it loses power while moving. It will take off fine and get up to speed then after a bit it starts to stutter. If I let off the throttle and take off again it will run for a few seconds or longer then refuse to take the throttle, basically bogging down like it isn't getting fuel or ignition. Looking for a way to figure out the issue without taking it to a marina (just dont have the money) Again thanks for thoughts, or common issues that m will make me feel better about ordering parts.

Factor: This was my father's boat and it say unused for years. He had to take the electric fuel pump out of the housing and tap on it to get it running again and it did for me for over a year without issue. I am leaning towards an electric fuel pump being the problem but am not sure.

Thanks again for any help!

[boats.net]

Best to have computer diagnostic run to check for fault codes, have fuel pressure tested when running engine.