Does this engine have the coils with the removable/plug in primary leads? If so, performing Mumbles’ tests is easier. Keep in mind that those primary connections are a big source of trouble, especially in salt water. Clean off the primary coil terminals, then pinch down the primary lead connector with needle nose pliers so it fits tightly on the coil terminal.
I am assuming this engine ran OK when you water tested it the other day, and this problem has just developed.

If not all ready checked, I’d say do a compression test
before going any further.

OK, so this is NOT the engine you were asking about cooling performance…
Like others have said, let’s not skip the basics so we can avoid confusion…
Well, the compression seems good…
You say it runs and idles great, but didn’t seem responsive….Seems like a bit of a contradiction….
You report no drop in RPM when pulling one of the plug wires, which would indicate a dead cylinder….You say the spark jumps your tester great, am assuming this is while cranking, not while running, is this assumption correct?
Do you have one of the peak reading voltmeters, so you can do running output tests?
If not, you could try those inline spark neon testers or a timing light to monitor spark while the engine is running. There may in fact be one dead cylinder, but the problem might not be spark related…I am assuming you have replaced the plugs, try different ones even if the plugs are new…Just because the plugs are new, doesn’t mean they (one of them) isn’t defective….
I just don’t want to lead you down the wrong path….Don

Don’t be afraid of this newer stuff, most times it can be diagnosed without fancy meters such as the PRV.
Again, you claim the engine sparks on both cylinders while doing your cranking test, is this correct? Did you have the spark tester set to about 7/16-.5" gap? You have replaced the plugs twice, so that rules them out…
Is this one a fresh or salt water engine? Check the coil and pack grounds using an ohm meter on the low ohms scale, anything over 1.0 ohms indicates a bad ground. Check those coil primary connections as well. Does the engine look as if is has overheated? If so, does the ignition wiring that lays on the block look melted or repaired? If so, perhaps there is a faulty splice/connection/repair that occurs as the mag plate moves the wiring during spark advance. It is a good idea to do the stop switch elimination test as well. Although a partially shorted stop switch would most likely affect both cylinders, never say never with electrical stuff. For this test, you will need the amphenol tools to push the black stop switch lead out of its rubber connector, please do not cut the leads to do this test.
If no problems are found during the spark test, grounds, stop switch elimination, visual wiring inspection, then I would use either a timing light or those inline neon spark indicators to see if you are losing spark to either cylinder when the throttle is advanced. Just be careful when using those inline neon testers, they can short to the block easily causing inaccurate conclusions. We can go on from there using Mumble’s simple testing procedures if you conclude that spark is actually dropping on one cylinder.
Finally, the only thing I didn’t mention was the removable spark plug leads themselves. Perhaps one of the plug leads is chafed, or has a lousy boot terminal connection, which is very common. I would pull apart that 5 pin amphenol connector and inspect the pins/sockets, sometimes they back out of the boots causing a lousy connection, or perhaps the pin/socket crimp has broken off, it is best to push all the leads out of both sides for a complete inspection. Use only alcohol to lubricate the boots/pins/sockets to easy disassembly. Using WD40 or other spray lubes will create a partial short circuit that will cause trouble down the road…..D