Just discussed this with one of our members. If the risers/ elbows or manifolds have a water leak, the water will flow down into the open exhaust port, by the valve, and into the cylinder. (If the motor has sat for a time--overnight is enough--water will have enough time to leak into the cylinder.) Since water doesn't compress very well (though it's ten times more compressible than steel), the starter will lock up as the piston of that cylinder tries to compress the water. This is NOT good!

How do you know this is happening? Your ear will tell you. Normally, you will hear "RRRRRRRRRRRR". But if there's water in a cylinder, you will instead hear "RRR--hesitate--RRR--hesitate". If you hear this sound, DO NOT TRY TO START THE ENGINE! Why? 'cause you may blow a piston, bend a rod, or etc. (My buddy saw the end of the block on a SB Chevy race engine literally blow off when it fired up with coolant in a cylinder.)

So what happens next? Remove the plugs and examine them. The "wet" cylinder's plug will be water saturated. With the plugs out, ground the wire from the distributor to the coil and crank it over--paying careful attention to the suspect cylinder. You'll see a spray of water come out. Finally, put the plugs back in and fire it up, to dry out that cylinder and prevent rust. Lastly--and most importantly--drain the entire exhaust system on that side to prevent more water getting in there.

Jeff

PS: If your engine(s) is/are cursed with those wretched swing elbows (with their rubber O-rings sealing the water out), then that's the first place I'd look.

PSS: A bit of detective work will help locate the culprit. Visualize how the water could have gotten into THAT cylinder, then look up hill from its exhaust port.

PSSS: Cold starting the motor(s) from the lower helm is the only way you're going to hear those starters cranking over. Right?