Hi . My water heater (6 gallons) installed on a chaparral sig 260 (bought last summer) was totally corroded and was leaking so I decided to replace it. When doing so I realized the red hose from the engine was totally clogged, meaning basically the heater exchanger was not in working order.
Question : is this exchanger use sea water or is it a closed system with coolant. Engine is mercruiser 350 I/O.
Thanks for your help.

This is the issue. I have no clue and was not able to find any info about that yet.
I mean the cooling of the engine is sea water. But Would like to find the chart of the engine to make sure there is not a closed circuit for the heat exchange.

Ayuh,.... Then the water heater uses the same water,.....

The circuit flows from a fittin' on the intake manifold, just behind the t-stat(hot-side), 'n returns to a fittin' on the circlatin' water pump,.....

Thanks.
Do you have any idea of where I could find the engine owner's manual... I don't have any.

Genuine Mercury & Mercruiser parts

If your motor is closed cooling, there will be a large round tube across the front of the motor along the top. cant miss it. Guessing 4" in dia. approx 2' long???? You dont need a chart of the motor to see if it has an exchanger. But that heat exchanger is cooled by seawater.
The water heater uses the same coolant (antifreeze or water) that is flowing thru the motor. The water heater itself is a heat exchanger, which is inside the water heaters internal tank. Only thing you canl see are 4 hose connections on the water heater vs 2 connections if it was a simple electric only heater.

Ayuh,.... If the op can't find a radiator cap in the motorbox, it's a raw water cooled motor,.....