The decal must be sealed before being put into water!
The varnishes available at hobby shops can be used.
If you are in the USA, one of the best available is Krylon Acrylic Cristal (available at Loews, Home Depot, Ace Hardware or Walmart) ,Humbrol or Hannants Xtracolor Gloss varnish work fine.
Laser decals are printed on continuous clear decal sheets and will need to be trimmed to fit.
Laser decals are transluscent and must be applied over light colored surfaces.