Unlike car manufacturer's one company makes the engine and another installs it in their boat. We think in gallons per hour first because that is what the engine manufacturer's publish. They don't know what kind of boat their engine is in. A 200 hp engine could be in a 40,000 lb trawler that uses 1/4 of that typically or in a 6,000 lb go fast boat that uses 80% of it typically.
It is up to us to convert the numbers that the engine manufacturer's publish in their prop curves to NM/gal based on how fast and at what rpms our boat can go.
Also there is no EPA mpg testing and reporting requirements for boats.
OTOH, Mainship did pretty good job, while they were in business, of letting each engine manufacturer test their boat/engine and report fuel consumption vs rpm and MPG. They were always a little on the high side of speed and MPG because the weight was minimal, but if you adjusted for that, they were pretty good.