I’ve never tried making my own decals, but it would kind of surprise me if you would need to spray laser-printed ones to protect them. The toner is waterproof — you can spill a drink over a laser-printed sheet of paper without any “ink” running (guess how I know this

).
IMHO the simplest thing to do is just to test it. Print a sheet with the decals you need, and add an extra bit to the sheet somewhere there’s room. It doesn’t matter what it is, just as long as there’s something being printed onto the decal paper. Cut that off, put it into water and see what happens. If the toner comes off, you know you need to spray the real ones to seal them against the water.