Same problem,88 Ocean 44, both turbos froze up over winter, corrosion on turbine side, caused by bad cast iron elbows. Removed elbows, water jackets non existant. Just received rebuilt turbos from Johnson & Towers. New stainless elbows being made at Marine exhaust in riviera beach Florida. Cast iron ells from J&T will not be available for three months. Stainless ells cost $2568. Turbos are around $2500.
J&T told me that bad elbows allow some salt water to hit the turbine bladesand casing raising hell with them. I loosened mine up with penetrating oil but decided to replace them since they are original, 23 yrs old, 2000hrs. iron elbows are $558 if you can find them.