Thanks for v much for all this great info!
Engine type: MerCruiser 5 litre Thunderbolt V Ignition (2001 boat).
He checked the prop hub while in the water. Me and his guy stood on the bow while he reached down to the prop. He was able to poke around that way. Since he could not confirm what it was - we're going to take it out of the water. (The damn tow boat at my marina is also goosed, so I have to get somebody down to tow me to dock ).
I did not hit anything while planing. I was moving through the water, just driving through some wake from another boat (2 or 3 feet wake), nothing major, and all of a sudden it appeared to slip out of gear. I couldn't tell if there was a 'breaking' noise, there was definitely a noise, but I think it was just the engine revving since the throttle was open and no gear engaged (it all happened quickly, my instinct was to shut the engine off immediately). There may have been other noise that I didn't hear/ notice in the moment.
Once I got it started again, I tried many times to get it in gear (FW or RV), as did the mechanic, we could not get the magic 'cluck' sound.
Thanks again for all the info.
Paul