I've been running Detroits since 1961. My favorite brand. Because they're reliable.
Maintain them, keep the oil clean, don't run a full throttle and they will outlast most yacht engines. I've got up to 11,000 hours out of similar engines.
There's gotta be several real Detroit Diesel mechanics in Fort Meyers. See who services commercial boats.
You'd be ahead to learn the maintenance yourself. Don't trust the average marina mechanics. The don't understand how the Detroits work, and most don't want to learn.