What caruses a 4.3 volvo penta boat crank case to fill with water?

Question

The engine runs fine. It is not missing like it would have water in a cylinder, but fill crank case quickly with water.

Answer 1

Sounds like you have a blown head gasket.

Answer 2

Head gasket - cracked head - cracked block - leaking oil cooler and, or depending on the type of water pump, possibly that.

Also, if a water cooled intake manifold, possibly that.

A head gasket does not have to be blown so that it gets water in the cylinder, causing a miss. It may be blown between an oil and a water jacket (passage).