'99 F100 L/U Issues? - Heavy Knock, Stalling

I have a '99 F100. At the beginning of the summer made it 10-15 mins out of my creek at low speed (engine normally running, developed an increasingly loud knock/vibration that appeared to be coming from the lower unit. When I attempted to throttle up the engine started to cut out on me with difficulty shifting between gears (caused the cutoff. Fortunately was able to fire it up, remain at idle speed, and make it back to the dock aided by the incoming tide. Had two different mechanics look at it and found that the lower unit was compromised, water leaking in mixing with the oil. After further consultation a decision was made to rebuild the lower unit, as it appeared to be salvageable (e.g. gears intact).

First time out on the water after the rebuild, plus a 100 hour service, nearly the same thing occurred. About 10-15 mins of running smoothly a knock started with vibrations. Attempting any increased throttle caused the engine to cutoff, couldn't shift between gears. Engine would only start up and remain in idle speed. I was ultimately towed in.

I'm waiting for the mechanic who did the rebuild and 100 service work to inspect it but in the meantime an thoughts about what might be happening? Thanks in advance for any feedback

Would check the lower unit oil. If more water, check what seals were replaced in the rebuild and whether the rebuilt unit was pressure and vacuum tested.

Take it back to the mechanic, If he did a 100 hour service and rebuilt the lower unit he needs to do it again.