What temperature difference will cause Exhaust Gas condensation?
This is typical of just not working the engine hard enough, or getting it up to proper operating temperature. The raw water inlet to the exhaust should be below the turbo unit and will not affect the turbo running temperature, however most marine diesels rarely get a chance to work hard enough to get thorough combustion. What you are looking at is basically unburnt fuel and the condensation of combustion gasses from the engine itself, check the thermostat to ensure that the engine is getting up to a decent operating temperature which will also help.
