we live out in the country and am lucky enough to have several barns/outbuildings on our property. We have always kept our boats/vehicles inside, especially during the winter month and, although i cant comment on rats, have certainly suffered as a result of mice/shrews.
Generally speaking though its the cars that suffer the most as mice, looking for a place to nest, tunnel into the engine bay soundproofing. We have found nests tucked in above the gearbox, inside battery boxes, behind headlights and have even driven 30 or so miles to see one jump out once we parked up at a friends house!.
The boats have suffered less though as we make sure nothing is left for them to nest in. Where possible we remove all cushions and other 'soft' materials giving them few options. I remove the covers regularly and open all compatments to make sure there are no uninvited guests. Once they get in, mice can be very destructive, knawing through cables, hoses etc...The good news is that boats are obviously made to be watertight so few access points are available to the little buggers.
I'l try to upload some pics of what they have done to some of our vehicles...
Although they are very agile, they do seem to struggle overcomming the overhang of the boat especially with the cover tightly fitted.
C'est pas l'homme qui prend la mer, c'est la mer qui prend l'homme....