My suggestion is the following:
1. Take a compression test of the rear two cylinders on both sides of the engine and compare them to the front cylinders as a cross check and a leak down test comparison would also point out any water (rust) damage to the valves, valve seats, cylinder bores or rings. If all is ok, you're still good!
2. You can slo use a bore-scope to look into the spark plug hole to view any valve rusting or bore rust or damage, if one is available for your use.
3. With your system, Eliminate long or low rpm idle periods with the Captain's Call system switched closed and do not run the engine above 1200-1500 rpms with the system closed also!
If you are water damage free now, you would appear to be in good shape and maybe just lucky. Hope you are damage free and you can enjoy your boat without the problem showing up.
Ray @ Raylar