I presume a Nautica 16 is 16ft long, or 4.9m.
We took our Ribcraft 4.8 with a 2008 Honda BF50 was on a run last year — unusual, because it's normally a sailing club safety boat and normally the third choice. About 5½ miles up the coast to another sailing club, all-day loiter with a couple of very short high-speed blasts on safety boat duty, then back home. Total fuel consumption was roughly 15L, (4 US gallons). Total distance probably somewhere between 12 and 16 miles, so we managed between 3 and 4 nautical miles per US gallon.
Given that the BF45 is a much older motor, it isn't going to beat a BF50 by a factor of 4.
The claim sounds like nonsense to me.
Edit: Here's some more back-of-the-envelope calculations.
28 mph ÷ 14 mpg = 2 US gallons per hour.
The density of petrol is roughly 6 lb per US gallon, so 12 lb/hr.
Assume that he's using ¾ power to do this: 34hp.
Brake-specific fuel consumption (smaller is better) is 12 ÷ 34 = 0.35 lb/hr/hp
For an early-90s petrol engine with carburettors, I'd expect 0.45 lb/hr/hp at best.
Assuming half power, the figure is a more reasonable 0.53 lb/hr/hp, but I'm convinced that you won't get a boat of that size to that speed on 28hp.