Other thing to be very aware of when reading replies - Nautical vs Statute miles and UK vs US Gallons. (At least Litres & KM are such a different name it's obvious the numbers are different!)
To expand on Nos' question -
2/4 stroke is one thing, but how you use the engine will give a much varied consumption for any pair of engines you might be comparing.
For example - if you blast out for 10 mins, drop an anchor and spend the day fishing, you'll use next to nothing. and you could use a 1958 engine with planet destroying emmissions and still see no perceptible difference in consumption.
Next stage - you blast out to your favourte bay & start trolling. That is where the difference between 2 & 4 stroke will be most obvious. Old engines throw way too much fuel in at low RPM - it;s the nature of the beast. (Also don't forget that new DI 2- strokes are up there with the 4- strokes in the consumption world at low RPM)
If you head out & spend the day razzing up & down at wide open throttle, the 2/4 difference will be negligible. Yes, there wil lbe a small difference, but real world unless you are out all day every day I doubt it will make a big difference to the dent in your wallet.
Now, you have a Zodiac 470. I guess thatls 4.7m long, so on a par with mine. If you do a search for "fuel consumption", you'll find on a rolling average most ribs will do about 1L/mile. Mine clocks 0.83L/ Nautical mile on a 20/22Knot cruise, bizarrely regardless of sea state. I did one trip down a canal (Falkirk Wheel 10th Anniversary - I was the only RIb at the event!) - 14 miles at 4 (statute)mph and I was chiselling the soot off the plugs when I got home. 2 L/nautical mile recorded.
If nothing else it's a snapshot to show that a premix 2-stroke of 60s design is actually more efficient at speed & Illustrates Nos' point!