Thank You For The Question XXXXXXXXXX,
I would say your issue is with the carburetor, probably fuel fouled with gum deposits..
We would replace the fuel filters every year, and the fuel pump would not be replaced unless proven bad, in your case I don't believe it is the fuel pump at all, at this point...
Thing is, the running of the engine on land requires very little power, and when in the water the power requirements are much greater along with the fuel requirements.
Typically the carburetor gets these deposits when the fuel evaporates from storage or old fuel is allowed to flow through the carburetor...
The prudent thing to do is a compression test and ignition test, (I don't believe you will find an issue ) but it is the prudent thing to prove the integrity of the engine and ignition system before jumping into the fuel system....
Marine Mechanic
21 Years Experience as Mechanic, 18 Yrs Boat Yard Owner/Mechanic, Factory Certified