how do you check the compression on a boat motor?

Answer 1

by using a compression tester

Answer 2

I bought a pretty nice one from the Wal-Mart. Any parts store will have them.
You remove all of the spark plugs, set the motor on full throttle, and screw the fitting into the spark plug hole and hook up the tester. You have someone turn the motor over a few times by turning the key or pulling the starter rope and see what the guage says.
The most important thing is not necessarily how much compression you have, but what the difference is between the cylinders. A 10% difference between cylinders is about as much deviation as you want. A 20% deviation from one cylinder to another is a problem and requires major repairs.

Answer 3

You can use a standard compression gauge available from most automotive stores, use to even be able to buy them at K-Mart. Basically you take out a spark plug, hold the gauge in the plug hole and turn over the engine. Gauge should be under $20 I think.