2.5 HP (starts in gear) propeller not turning?

We have a Nissan 2.5 HP long-shaft 2-stroke on a 17' sailboat.
The sailboat is used by many different people...it is club owned.

One user is reporting that, after the engine is started, sometimes there is no thrust....even when the throttle is opened.
Other times everything is OK.
I would think that is next to impossible.
If the engine is running, the crankshaft is turning....the driveshaft should be turning (I know there are no splines...just a square tube).
The gears are always engaged...the propeller shaft should be turning.
The shear pin is fine, the propeller should be turning.

I am not aware of any very hard impacts.

So, what is going on? What is the most likely cause? Is this a known problem with older Tohatsu 2-strokes?

P.S. There is no neutral on this engine....it starts in gear.

You must have an old 2.5A.
Are there a lot of weeds in the harbor?

Yes...it is a NS2.5A....excellent engine.
I think you hit the nail right on the head.....SEAWEED!
Another user reported difficulty restarting the engine.
I had seen this before, but since I wasn't there this time around, I didn't think about it.
I believe the engine is 100-percent okay.