If it really is steam then it is water
of some kind getting into the oil. In addition to coolant
it could be seawater that backs up into the exhaust
turbo and into the crankcase in between outings.
Check the underside of the oil filler cap for brown milkshake sludge and/or oil level rise.
But it could be unburned diesel
that blows by the rings. Somewhat normal depending on severity. Does the engine
start easily from cold. If it doesn't that is a sure sign of excessive blowby due to worn rings.
You can do a "red neck" blowby test. Run the engine under load up to cruising rpm
. Take off the oil fill cap and hold your hand over it. If it gets blown back due to heavy vent pulses that is excessive blowby.