Perplexed - running on 1 cylinder (sometimes)

I'm slowly learning the ins and outs of outboard service (the hard way, apparently). I'd certainly appreciate any insight or thoughts on what I'm dealing with here...

I have a 1994 25hp Merc that has been running on 1 cylinder. The first time it happened, I put it in saltwater and the cylinder was firing intermittently, it was in/out and then settled into firing fine and I ran around that day with no further problems. Next time, took it to the coast (saltwater) it cranked up fine but only 1 cylinder was firing no matter what I did. So I took it apart, checked compression, gaskets, plugs, etc., took it to the lake and it ran great. It ran great for several trips to the lake, screamed like a banshee. Took it this weekend back to the salt, and again, only running on one cylinder. Pulled the plugs, cleaned em a bit with a file and then it was intermittent. Aha! I thought, got a bad plug. Played with the leads and found it was the top cylinder. Replaced with brand new plug. Still ran intermittently for a bit, then finally kicked in and ran good after kicking in and out for a bit.

What the h*** is going on here? I doubt the water (fresh/salt) has anything to do with it, probably just a coincidence that it keeps failing in salt, working fine in fresh? Unless some sort of electrical stuff is going on... I don't know.

Where would you start next in diagnosing the issue?

You are pulling plugs to gain.
Either connection at plug or excess fouling(?) , like chunks of carbon plugging gap in plug.They can drop out before you get to see . Any view to top of piston?
Your not overheating from your description.The only other thing that comes to mind is a cracked coil/power pack working intermittently due to dampness.(When checking, the back sides need attention too,for hidden trouble.) And somehow when working on plugs, maybe a drying effect on a cracked ignition component aired out from engine cover removal, it changes symptom.Electrical seeming prime suspect.

Now that you've identified which is the failing cylinder, switch the coils and see if the problem follows the coil. If the top cylinder continues to be the one to fail after the coil switch, it eliminates the coil as a possibility. If coils test good, replace the plug wires. They aren't that expensive and if they're original, it's time to replace them anyways. Hopefully, one of these things fixes the problem because having to replace the switch box or the ignition module can get pretty expensive.

Thanks guys, I'll do that and see what it gets me.
Hopefully it'll isolate it.