Which engine have your got, is it the 115 4 cyl 2 stoke ? if so they weren't the best engines mercury ever came up with, but you ought to manage about the 1lpm rather than 2.5 lpm.
That petrol is going somewhere, so leaky fuel pump ? or carbs, fuel lines etc. Probably worth a trip to the dealer to solve this one, with a full service to include timming and compression check etc. It should pay for itself very quickly.
JIY, the Pacific does approximately 1 lpm, @ 25 ppl
so thats the Scillies and back for £20 (sorry Rob)