For 260hp petrol four-stroke work on something in the region of maximum consumption 18 imp gallons (83 litres) per hour at wide open throttle if propped properly and developing full power. What happens at lower speeds will depend on hull, weight and so on but cruising speeds typically half to two-thirds that amount. V8s tend to be thirsty motors, so I doubt you'll get much change out of this.
For fuel consumption rules of thumb see 'Any one with a Day Angler 21' a few threads below this one.
Hi Alfie - last year we managed about 17gph on a 260HP Mercruiser 5.7 mated to an alpha one outdrive. This was a lot of slam-bang stuff and tides / wind rubbish that brough the cruise down to 17 t o18kts so with a better riding boat (or better conditions) I'd hope to be nearer 15gph at 20kts.
For the moment I'm assuming that my engine will be burning go-juice at the rate of a gallon a mile on the plane and about 2/3rd pottering elsewhere. I hope to be pleasently suprised but the only one that is likely to be is the owner of the fuel barge!