What's important in a boat engine is not peak hp that you'll seldom use, it's how much it puts out at the cruise/max continuous rating (at about 5"Hg manifold vacuum) and how much mid-range torque it puts out for climbing on plane. There is typically no substitute for cubic inches for that purpose. I'd pick a 350 ci V-8 over that 4.3L V-6 in a heartbeat. The 4.3L might be ok in a 20, but not a 23, which is a MUCH bigger and heavier boat. A V-8 will also be a much smoother running engine than that 90 degree V-6 which is not a naturally balanced engine like an I-6 or V-8.
A good friend of mine owned a Moesly 21 for 27 years and was on his 4th engine when he sold it. (The SeaCraft 21 is similar in beam and deadrise but lighter than the 23, with better ride and performance.) He was a gung-ho diver and ran it year round every weekend drift diving off Palm Beach carrying 4 divers + gear, and 8 tanks. It came new with a 150 hp I-6 Mercruiser with marginal performance; cruise with that heavy load was about 18 mph @ 5" vacuum. He burned out exhaust valves and cylinder heads often until he learned the importance of observing the 5" vacuum limit. He replaced it with a 165 hp I-6 Mercruiser which picked up cruise speed 1-2 mph. The next engine was a 260 hp Mercruiser V-8 that would easily cruise at 22-23 mph at about 9" vacuum, with much better life on the exhaust valves and seats. His last engine was another 350 chevy, but a Volvo I/O version rated at 290 hp and it was his favorite of the bunch, with a WOT speed of about 50 mph!
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