This is a bit of a "touchy feely" reply, but I once ran a Mariner 40 2-stroke on the back of an SR4 duting the Oppie nationals and managed something like between 9am-5pm on 3/4 (ish) of a 5 gallon tank - 6 days in a row (as in 3/4 tank / day!) . Like you suggest, a lot of tickover pootling, and a few blasts to / from the course & when someone capsized.
Can't be a lot of help on the 4- stroke I'm afraid.
The trick for stuff like that is to switch off when not moving, and if you're lucky, the tide / wind will work in your favour as well! Also if you are on premix with the mariner, will stop it oiling up.