Disconnect the shaft from the trans. You should be able to turn the prop shaft by hand. If it requires a pipe wrench to move with much force, then you have a shaft problem. If it turns freely by hand, then it's an engine problem. Lack of power under load is usually a fuel problem. These guys work with nothing but A-4's, maybe they can help also. Google-ing Atomic Four Marine Engines, brings up a wealth of info as well.
Moyer Marine Atomic 4 Engine Rebuilding and Parts