If you choose a waterslide decal, give it a few light mist coats of clear lacquer before use in the water etc.. Same after applied to headstock although I think some old Fenders just had decal plonked on top of final lacquer coat.
I applied a decal to a P bass copy headstock this way and it looked as good as my real one.
Good luck with your choice of design and put up some pics when finished.