PRM 160 box is damn near indestructible providing the oil is changed and there is no water in it.
The base engine is good. I'd rather change a belt than a timing chain like on a BMC 1.8D, its a simple job, even garage mechanics mannage it!
We ran lots of vans with belts without any problems even when they were used long past the due replacement mileage. We used to inspect the belt, If the writing printed on the back was still visible and there was no tooth cracking, and tension was OK we left them alone. Its easy to check regularly if you cut a small part of the side of the cover off.
I would think that in a boat, running this engine way below normal power and revs, the belt will last years and many hours. If the boat suits you, I would not be put off by the engine being a Ford rubber band.