Andy, heat those bolts up with your benzomatic torch.

dale

get’em wet don’t let’em set!

Well, considering that you say the engine runs just fine once freed up, I will hazard a guess the head gasket/eroded cylinder sleeves are not an issue…But who knows. That leaves the lower seal and exhaust cover as suspects. I had a very interesting case once. A dealer reported a constant water leak into just one cylinder of a newer 88hp, even after new head and exhaust cover gaskets.
I decided to remove the gearcase and run the engine a little bit to warm it up. I then attached a hose to the water tube and just ran water through the engine while I peaked into that cylinder with an inspection light….I noticed that water was dribbling into that cylinder through its exhaust ports. We pulled the exhaust cover again, everything “looked OK”, the dealer told me that the inner exhaust cover was actually new, he had replaced it during his last attempt to solve this issue. We found the block casting was actually porous around one of the ports allowing water from the cooling jacket to leak in. Needless to say, the only way to solve this was to replace the powerhead, but at least the mystery was solved. This was a newer engine with a casting flaw, had nothing to do with its salt water environment.
So, you may want to try pulling your gearcase, and running water through the powerhead with a hose attached to the water tube. You will need a good inspection light, perhaps one of those inspection cameras, and much better eyes than mine. Once you have spotted the leak, be sure to run the engine a bit to expel any water. Needless to say, this method will not point out a leak caused by a bad lower seal though.
I agree, your spray lube isn’t even reaching those stubborn exhaust cover screw threads. Like you say, the caked up salt has sealed up the screw shank quite well. You can try heat, but those darn screw heads get buggered up pretty quickly. You may just want to drill the head of the stuck screw(s) right off so the cover can be removed. Once the cover is off, the caked up corrosion will no longer be an issue. You can grab the protruding screw shank with vice grips, apply some more heat and patience, the broken screw stud will probably give in and come out for you.
Good luck, I know how these mysteries become our most important missions sometimes…

The 18 volt Milwaukee cordless is what I have, too. I use a Snap-on bit, though. I forget what it cost; but it’s probably in the $35 range. No cheap; but it’s the best there is. Harbor Freight may have a great guarantee but sometimes, you would just rather get the job done, rather than go back to the store over and over. (Especially, in THESE times.) I also used Mapp gas on my salty FD exhaust bolts.

How do you like breathing muriatic acid fumes? I used some of that, to get what was left of a thrown Tecumseh snow blower engine connecting rod off a crankshaft. It works great; but, even outside with a light wind, I couldn’t believe how bad the fumes were!