[Fr. Frank]

I spent over 20 years in the marina business. Keeping an I/O in saltwater is only a problem if the maintenance isn't performed properly, particularly sacrificial zinc anodes and outdrive impellers, and fresh water flushing of the motor after use. Otherwise, no biggies.
Clean your manifolds and risers every 3 years, and then replace every 10 years, or just replace them every 5 years. The cost is about the same, but the first avoids the deterioration in cooling performance occurring from years 3 to 5.
Raw water cooling DOES have a deleterious effect on the block and heads of the motor, as well, but those are generally long-term issues (20+ years) if regular PM is performed.