I have been using synthetics for decades in my truck, which has an International engine, and my JD tractor, which has a Yanmar engine.
The truck oil is supposed to be changed every 5,000 miles. I change at 15,000ish. Friday I changed at 16,000ish. I get a UOA done and last years test said to try 17,000 miles but the weather was nice and I had time to get some chores done so the oil got changed "early." UOA over the decades has said I have been throwing away good oil even with the high miles/hours on the oil. The worst the oil has even been was during a time where I was getting stuck in bad stop and go traffic. Fuel was in the oil from the extending idling BUT it was still within spec.
Why do I use synthetics? It saves me time and money. I get slightly better MPG, when it gets cold I don't have to hook up an engine block heater which really drives up my power bill even using a timer, and I only change the oil and filter once a year instead of three times.
With UOA I know how the engine is wearing.
Is it worth it on a boat?
If you use JD synthetic in a JD engine, you can increase the oil change hours to up to 500 hours instead of say 250. That might be worth it for some and not for others.