Then you try and tell me that water can only get into the exhaust when
the boat is "seriously rocking for some time".
What is this "seriously rocking"?

that is rocking through an angle fore and aft to the point that water in the
exhaust thru-hull is tipped and rolled forward enough to rise above the high
point of the exhaust outlet hose coming from the water lift muffler. It is
also rocking enough times to fill the water lift muffler from reverse and then
fill the outlet hose to more or less its high point to push the water in the
water lift muffler back into the engine exhaust manifold and push the air in
any cylinder with an open exhaust valve past that cylinder's rings and then
fill the cylinder with water. On most boats with a properly installed exhaust
outlet hose this is not a problem as the boats are normally used. In some
special cases the installation does also need to prevent water ingress due to
rocking.

Got anything to do with being on an ocean?

not really, through rocking conditions sufficient to water ingress are more
likely on the ocean that fresh water, ocean water is not necessary.