For almost all leisure boat owners, 60/40 is exceedingly generous. We've just spent a winter living almost full-time in the marina - most of the time there were about half a dozen boats occupied out of several hundred. Most of the mobos have not seen their owners in the last five months - but they will be back in late March, filling their tanks and claiming 60/40 - there is no way that they will be using 40% of the fuel for heating - and they are on shore power, so they are not using it for generation either - it is simply tax evasion.
Not that I have any problem with tax evasion of any kind - I'm all for it. But when you indulge in a bit of tax evasion, you need to remember that it will not last for ever - think yourself lucky for whatever you get away with but don't complain when it gets blocked off.