I concur with Phil. Old decals both disintegrate when in water for a long time, and the water soluble glue takes much longer to release. Further, even if you do rescue the decals, the glue may not hold very well when everything is dry.
One can prevent the the disintegration by coating the decal with a clear solution sold for that purpose. There are several brands available at the hobby shop. A decal solvent may help the adhesion problem.
In this day of inkjet decal paper, I always scan decals of very old kits before I try to apply them. If the decals do get ruined, I may be able to duplicate them.