Beamish is probably right - if it starts it - its probably OK.
He makes the point that some VOCs may be lost, and that they affect starting. Some might also offer some 'engine protection' properties or some other sales spin. I suspect they don't do much in an OB vs a Car.
How full is the tank? A full tank has less air space, which means potentially less evapouration. But also less moisture in the air which will potentially have been absorbed into the fuel.
It looks like you only run a 20HP so we are presumably looking at <40L of fuel.
I'm assuming its a 4 stroke (or at least not pre-mix 2st). Do you have a petrol car? If so mixing a wee bit at a time with your car will get rid of it and doing it bit at a time would avoid any losses being too important. Its probably not necessary - BUT if you are 3 miles off shore is that the point you want to discover an issue?
__________________
|