So, you say it will move the boat 2 MPH. That tells you that it is going into gear. I say you have a spun prop hub. Just a coincidence that it picked right now to happen. To prove it, make a mark on the prop nut (or just use the cotter pin as a mark), and make another mark on the propeller. Run motor till suspected slippage occurs, then stop and check the marks. If they no longer align, the hub slipped. If they do align, repeat the test. There is a chance it stopped slipping in the same place.
If you find it is slipping, simply replace the prop.