Low compression causes.

Besides rings,head gaskets, and reed valves, what else could cause low compression?

1. Condition of the cylinder bore.
2. Compression gauge, type of. or Wrong check valve (use of a tire valve)
"Reeds" will probably incite a lively discussion.

When compression starts in the cylinder the reed valves are NOT part of the picture.----So reed valves have nothing to do with compression in the cylinder.

A hole in the top of the piston will do it. I can't say I've heard of it happening much on outboards, but I've seen it plenty on jet skis. With a carb per cylinder one can get partially plugged and starve that cylinder for fuel. The fuel air charge is what helps cool the top of the piston and when it gets too lean the heat from the spark plug gets localized and melts the top of the piston creating a hole. Most people will tell you the engine ran amazing right up until they heard a pop, clatter, and no power. It will look something like the attached image.