Back around 1980, I installed a floscan system to a single engine 204 Grady and loved it. I had a 4-cyl in/out board that seemed to make gas because it only used around 7 gallons/hr at W.O.T. (around 25 mph). Although I loved it, it was of little use because I used somewhere between 5 and 7 gallons/hr no matter what speed I ran. It was a gadget that I like to fool with.
When I got my second Grady, I installed one on it. It had a 225 Johnston on it that sucked gas faster than the pump could fill the tank. I had sticker shock when it read 15 gallons/hr at 25mph. I really needed it to fine tune the speed for the water conditions and load on the boat. Properly trimmed it could save me 4 or 5 gallons/hr. This was a flo-scan meter also.
My third Grady has twin 250 Yamaha 4 strokes that have the gages that give me the total flow for either engine or both if set up that way. I have also installed two Garmin 4210's that are attached to the engines and they also give fuel flow of either/both engines but in a much clearer, easier to see format. I now seldom look at the Yamaha gages because on the 283, you sit further back from the gages and they are harder to read for an old person. One of the 4210's are always on the engine gages to show fuel being used, speed (GPS), RPM, total fuel used and oil pressure. I doubt I could function without this information readily available.
The Garmin system is usually right with a tolerance of less than 1%. Twice when I filled the tank up with more than 100 gallons, the Garmin system was within a gallon or two and this can be explained by maybe the nozzle cutting off early or the temperature of the fuel.
I cannot explain why, but the Yamaha gages are accurate to within 3 or 4% which is not as good as the Garmin but I'm sure both use the same metering device, just report the results in a different format.
If you do not have a flo-meter on your engines, I feel it would be important for you to install one. The only experience I have is with the flo-scan system and it was a good experience.
In addition to the possible savings you would get by monitoring your fuel use, there is a safety factor you need to consider. I do not trust any fuel gage that depends on a float to determine the amount of fuel in a tank.
I hope this helps.
Ed