You say that like spark and atomised fuel in the bilge is a bad thing....

Dont listen to this joker,,,if you get the concentration just right you will no longer have hydrolocking cylinder problems....

on some of the 496's if the exhaust is to low and you pull back your throttles to fast I've seen water get sucked up right back into the engine and hydroloc them

intake leak almost always puts water in the oil or to the outside so it's likely not that

I'd pull the sea water pump belt while you crank the disabled motor. Cranking, without starting, can suck water up the exhaust if in the water or on a hose.

tommy,disable the ignition&fuel pump,like BUP said it is not a good idea to crank it with plugs out and system,s on.also,don,t put wd40 or brake cleaner in the cylinders.USE MARVEL MYSTERY OIL.

100% correct.the only way water can enter is through the exhaust.the block and heads have a antifreeze-water mix,not straight water.

Just disconnect the large connector on each side for the coil packs. That will disable spark and they are easy to get to.

troubles with small amounts of water in cylinders and on threads of plugs. And no apparent water leak in manifolds, heads, intake. Then problem may be reversion, reversion, reversion. Fighting that right now. Severe case in my boat. Big blocks have a lot of this issue. Mine are stock and just rebuilt both 2 years ago. pulling the heads already (only 2 years old with normal use due to reversion. Hopefully from just the valves, not bottom end. Merc has faught this forever. Look up the exhaust after idling and if you see many little light colored specs, thats the traces of water/salt water that is backing up in the exhaust when idling from REVERSION. Merc has made a "resonator" to put up the exhaust to correct this but may cost you 1-2 mph. But if it saves motors, worth it. Other better alternative is to weld on tubing to the elbows to get the end of the exhaust tubes further downstream before mixing with the water. Also slash cutting the tubes will help performance vs flat cutting them. A little bit of work but it would have saved my valves after only 2 years of service. A friend of mine has just done this instead of the resonators and completely stopped the reversion, and because of not using the resonators has lost no top end. I am going to do this on mine in next few weeks. I have also found just last weekend that the elbow temps are way to cold in fall, winter, spring even on the gulf coast where i live. THey are running 105 to 115 degrees and the manifolds are even colder. Causes bad "condensation". You can look up the exhaust pipes at the top forward curve of the elbow after stopping and see the water condensing. Need to stop that from happening. I am going to restrict the water flow to the manifolds to raise the temps after talking with Merc today. Simply put in a PVC valve and tinker with it to get the right temps without so little water of course that you burn the hoses. It doesn't take much water to keep hoses cool. If you need more water thru the block, just dump some overboard instead of restricting it. Again, put a valve to control it. Like i said a little work, but these motors especially mild ones, should run for many many years without rebuilding if corrosion does not get to them. Unfortunate, but no way around it. See many people with this problem that never realized what was causing it. Hopefully it may not be in your case, but thought i would share what experience i had. I just put in the resonators to try them temporarily but like i said, i am redoing the exhaust shortly. Some high perf manifolds may address this issue but no matter what brand i would inspect for reversion anyway. Headers should solve the reversion issue but they are a little pricey, and it not proper application wont help performance. REVERSION AND CONDENSATION .... not checked often enough and should be one of the first as its easy to do by looking up the exhaust with a bright flashlight. Get them corrected.

Turned out whomever worked on the boat and installed new manifolds before they bought the boat removed the flappers in the y- pipe . Reversion is what the cause was according to a Merc mechanic friend of mine who is familiar with 496s and repaired it.

I had this happen last fall in my 496. #6 cyl. Flappers in Y-pipe had broke loose and fallen down.