You could look at it this way........just put a freshwater flush kit on your engine and flush it everytime. My '94 Formula had 10 years of use on it in all salt water with the original engines (twin 502's) and was still going strong. The manifolds were replaced after 7 years.
So even if you ran your MCX at least 5 years in salt water (using flush kit) you could always replace the engine for alot less than $7K. (prob less than $5K)
Add zincs to the rudder and shaft too. Oh and don't forget WD40 is your best friend!