Whichever exhaust pipes are the easiest for water to flow into would have water in the associated cylinders. I have never seen the internals of an Indmar manifold, but I would guess they are symmetrical.
If the two wet cylinders have the easiest flow path, it would make sense that only those two would be wet, and on both sides.
In the scenario I am using, the water would have to be coming in each exhaust port, at the same time. I cannot imagine any scenario that is more likely.