Many have,, both on Ski's and boats.. I would go direct to the battery with an in-line fuse so that you do not draw from any of the electronics of the ski/boat itself.
my bro in law did one inside his glove box, he was already in there wiring up the gps so he just added a usb port and mounted it on the side of the glovebox so water wouldn't get inside if the glovebox got wet.
FWIW, I just bring one of these along with me when I plan on using the phone a lot on a trip.
$15 shipped and it's good for two full charges. (I have two of these and the 5000 model)
added bonus that you can use it on vacations, planes, trains, automobiles, etc.... (and it charges faster than my wall plug does)
I liked mine so much I bought one for my daughter, mother, brother and my best friend (all 4 of them constantly were telling me that their phone was low)