Personally I find that the worse the ocean state, the easier it is to just eyeball the rounds in just using the crosshairs. After a while you can get a good idea of where to aim for certain distances, speeds, etc. Of course, if the weather's good enough, then the optics are the way to go.
Although, if your shots are going ridiculously far left/right then your deck gun may be bugging out... I remember a few people complaining about that a while back. I think what happened was the deck went under while they were traversing the gun, and that threw off the gun's aim somehow, causing the shells to go waaay off-target. I can't remember the solution though.
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