I have a small water leak, the oil turns to milkshake after about 10 hrs, I removed the oil cooler and pressure tested it ok, and then pressure tested the block, (an old 400) and it held fine, 15 lbs. I listened closely to the engine and it runs fine.

how did you pressure test the block?

what color is the oil, really white or still dark? how often do you use the boat?

Check the intake manifold gasket and the riser gaskets on the exhaust manifolds.