Fresh Water Cooling system leaks

[Mike Sturgis]

I have a 1990 Merc 5.7L I/O with the fresh water cooling system (heat exchanger).  The issue is that the coolant (just water) in the block leaks out over time or during use.  I can "fill" the block by filling the heat exchanger and expansion tank.  I am debating on removing the fresh water cooling system or having the heat exchanger rebuilt.  Has anyone seen this before or converted from a fresh water cooling system?

 

Thanks