The local harbour pilot is as helpless as everyone else on board when the engine(s) fail. Maintenance inspections and lower speeds is what l forsee. Or talk of it anyway.
I just looked at my 22 F350. 275-65/20s on the ground. 275-70/18 spare. 20-40 extra revolutions over a mile isn't going to hurt anything. Half the vehicles around you have a space-saver spare. Spinning one wheel excessively in the mud/snow or giant peg-leg burnouts wreck spider gears.
We're talking about a spare here. In the off chance he has to use it the spider gears aren't going to care about a 3% difference. I'm pretty sure he drives a newish Ford which should have a selectable locker. Matching the circumference with a limited slip does matter though.