73 Johnson 6hp. Fuel or Driveline?

When in the water in neutral I have full throttle response, when in forward or reverse, barely more than an idle. If I pull the prop out of the water, I have full throttle response. Seems like when under load it doesn't generate enough power. Is it not getting enough fuel? Bad mix? Gear oil is fresh and full.

In neutral OR with the prop in gear BUT out of the water... it would seem that you are getting full throttle but you really aren't. It just seems that way as there is no resistance to the propeller (load).

You need to check the compression psi of each cylinder first, and second (with the plugs still removed) the spark which should jump a 7/16" gap with a strong blue lightning like flame. A real SNAP!

If you have proper compression and spark, the usual problem is clogged carburetors which need cleaning and rebuilding.

Let us know what you find in the compression and spark department.