Compression Diagnosis

[boodlehound]

I have a 1990 2-stroke 90 HP Merc that starts and idles fine, but cannot get boat up on plane (not enough power). I had compression test run on all 3 cylinders at boat mechanic shop. They said there ws no compression in the top cylinder and I needed a ring job. But, an aquaintance said it may be that I need to replace the head gasket. From what I have read, bad rings will not allow the engine to idle or run smoothly at low speeds and will make it hard to start. This is not the case. It starts fine and only lacks power at high rpms. How should I go about finding out which is the correct solution?

[boats.net]

That engine a closed cylinder style block and doesn't have a head gasket. Low compression will result in loss of power, may still idle on 2 cylinders but not good.