Problem with prop staying engaged

A couple days ago i went on a fishing trip here in nebraska and boated for two days. On the third day went to new lake and put in, took off across the lake and everything was going fine then all the sudden the boat lost speed and the engine reved up. I throttled back and then tried to go up to speed again and it did the same thing. It would idle and the prop would turn fine so idled back to dock and loaded up with wench because couldn't get enough thrust without it going out again.

My question is what the problem would be.

Ran boat all last year and no problems or shift problems

Spun hub in the prop methinks

I agree but for it to happen as you said, you must have hit something. Plastic replaceable hubs can shear and not recover. Pressed in rubber inserts are built to, and usually slip when an obstruction is hit. Reducing speed to idle, and putting the shifter in N and then back in gear and bringing RPMs back up it will normally reset if this is a seldom occurrence. Over time this may degrade to the point where it just slips when you give it the throttle and its time for a rehub or new prop.