Re: motor reving very high....boat barely moving.
It is more than likely a spun prop, but to throw another experience into the conversation: After working on a 135 Evinrude extensively to bring it back into service and testing it on a nearby lake, every outing was filled with apprehension. There were two mitigating factors in this story. One, the water pump had not been replaced and the lake was very shallow and weedy with an extended No Wake from the launch site.
The motor powered up and as it reached higher rpms the boat slowed to nothing, back off the throttle and it would proceed at a slow troll. When the motor was powered up again the "spun prop" syndrone reappeared. At that point, the hot alarm sounded. After shutting down the motor and using an electric trolling motor to get back to the ramp, a spun prop and a failed water pump was all I could think of. After disassembling the LU and looking at the impeller, I found the impeller intact - worn and in need of replacement as a maintenance item and the hub in the prop was ok. What I found and what happened on the lake did not square until I remembered removing a huge string of seaweed from the lower unit that blocked the water intake ports - hot buzzer; and the seaweed caused a cavitation effect on the prop.
Long story short; it's OK to default to the obvious, but there can be other conditions that contribute to a problem.
