Hi Fred, you can use waterslide white transfer paper, if you can match the background colour to the part of the boat you are applying it to. I have done this a number of times and it can work quite well, (see lettering on fuselage of plane for eg) I used a stencil type black lettering (you can remove this later) then flood coloured the background red,- once finished, sprayed etc, trimmed closely and applied, (you only need to trim the outside excess, not the individual letters, - although I did do that with the stars as they were easy to cut out).
Takes a bit of mucking around but can be done,- I just used Windows Paint which is pretty basic, - a better system like Corel might be easier. Just make your background colour in a small sample, print and compare etc till it's spot on, (allow for darkening with varnish or lacquer, - test spray your closest sample before printing final decal)
I just did it as a stencil first, but changed it later to the white lettering, as it stood out more. (pics)