Is it safe to assume that this problem occurs in both forwards and reverse? Will it stall at lower throttle settings or only higher throttle settings?
The way that you describe it, it sounds like you are putting a large load on the engine. The suspicious part to me is that the engine stalls when you try to rev it up (I am assuming that it doesn't stall at lower throttle). It is possible to overfuel so heavily as to cause the engine to shut down but that is pretty extreme and would cause a very large load.
Possible loads could be from the prop being severely overpitched, running in a slip with a large current
from astern, a fouled prop/shaft (line or fishing
line or something else), a binding shaft (does it wobble or vibrate in gear?), or an internal transmission
Before hiring a diver or hauling, you could detach the shaft from the transmission
and try revving up in gear that way. If it loads down, you need to suspect a tranny problem.