Usually the rating of the engines will show the style of cruising service the boat was designed for.
A 36 ft boat with 100 hp is going to be really cheap at the Sq RT of the lwl, say 6k for LRC.
Hull speed will double or more , the burn for an extra K or 2.
If the 36 ft boat boasts 350 hp, it was designed as an inshore weekender , the speed will be double the small engined boat 15K but the fuel burn will be 10-15 times higher.
On unusual boats , where you cant simply ask owners , a quick math will work.
Find the vessels displacement , that is how much it weighs (not the USCG "tons" which is volume).
Divide the weight in pounds by 2240 for the displacement in tons.
Slow cruise will cost 2 or 3 hp per ton, so multiply the tons by 3.
Then divide that number by 16 for a diesel or 12 for gas.
This will give a rough GPH at normal cruise.
I think you missed the mark a little on fuel consumption for your hypothetical 350 hp vessel Fred. For example, my 420 hp single engine boat burns only twice as much at 15 knots, compared to 8 knots. Not 10-15 times the slow cruise. I get just over 2 mpg (assuming no tidal effect), at 8 knots, and 1 mpg at 15 knots. This is factual, not incorrect theory.