no, no rocker cover... no rockers... its a flathead, cam & valves are in the block.
ok, I'm assuming that the problem child valve is stuck open, on the cylinder closest to the rear of the boat, making it almost impossible to access.
The "access" hole the mechanic wants to cut may not be all that bad of an idea, unless you feel real comfy pulling the motor.
Depending on the availablity of space and patience, you might want to try these...
Buy a head gasket set and remove the head. Tap the offending valve on the head with a soft hammer (or a hard one and a wooden block), rinse and repeat. (spin the motor over till its free)
The valve train access is under the intake and exhaust manifolds. If you have long enough arms, you might be able to remove those and use a screwdriver to pop the valve down.
Sounds like you've got at least one weak valve spring. I dunno, the mechanic in me says replace it, the sailor in me says, If I can get it free enough, it'll loosen up just fine, remember, you're not building an engine to take you at breakneck rpms around the brickyard.
The ATF and marvel oil are great ideas, but you gotta get it unstuck first.
We are not primarily on earth to see through one another, but to see one another through
Some people are like slinkies: not really good for anything... but you can't help laughing when you push them down the stairs