well first things first, check compression with engine hot. see if your losing compression. don't worry so much about different compr readings (unless say one is at 125 and another is like 80), be more concerned with difference in 2 adjacent cylinders usually meaning head gasket leak. check all cylinders are firing.
is primer hose and bulb in good shape? pull connection off engine side of primer hose and check the inside of hose for deteriation.
easiest way to rule out problem with boat fuel tank and lines is to run engine from a portable tank with known good components.
if you have problems with engine fuel pump etc, no matter how far the throttle is, the engine cannot run any faster because it can't get more fuel to support going any faster.
could even be an issue with stator, as sometimes as they heat up they start failing.
easy stuff first tho, rule out fuel system and spark.
