The intake valves go tight, that causes the loss of compression. The first symptoms are hard starting. The cams have auto compression releases, so the compression is low when testing; something like 50-70lbs is normal. 20lbs is not enough to start.
It doesn't explain why it stopped dead.
2002 665 Prairie
FPS 12:1 pistons
FST Stage II Cams
Other weird stuff
2011 Stratos 186XT Bass Boat
Mercury Optimax 115hp
Minn Kota 70lb 24v trolling motor