With the advent of desktop publishing software and tools for cutting vinyl, there are plenty of sign shops that can do this type of work these days.
I don't think you need to go to "Marine" outlets for this.. but do let the supplier know where you plan to use the decals. We had good success with a general sign/banner outlet and nowadays even custom graphics are much more affordable and easier to produce.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)