Actually, looking back on a lot of years of kits, I can't recall ever seeing instructions regarding decal application, other than where on the model they were supposed to go. Maybe just a mention in a "hints and tips" section in a magazine or tips on a forum like this.
But waterslides do benefit from being applied to a nice smooth surface and do need waterproof protection. Provided that the makers knew what they were doing, a white decal (really difficult to home make due to there not being easily available white ink) will show up very well on a dark varnished wood background.