Volvo Penta 5.7 problem, I'm new to all these

[Ramp120]

The problem I'm having is that after using the boat for about 60 to 90 min the rpm goes down to the point that the engine chokes, when i turn it on again the rpm's are still low and when moving the shift foward it chokes again, that the only thing I can do is wait for 10 min to turn it on, them accelerate the engine with cable under the carburator for a minute and I have some one else moving the shift to move the boat........... I notice the water that ones out once I have the boat on trailer is way to dark, looks like mix with oil...... Ay idea what it could be??