This occurs if the decals are old or were storaged in a wrong way and for a long time. Also I have noticed that it could be a problem with the composition of the water in other countries. Here, I mean Poland Czech, this happens really seldom. Actually there is no help if these were shattered in the water. However if the pieces are enough large you may try to pick them up separately and move on the model where you can piece the markings together. Usually decal sheets should be covered with a gloss clear coat if you are not sure of their "Life history". This prevent against taking decals apart.. Certainly it has to be done before soaking with the water.
Well ... now you have to try to obtain a new sheet of these decal markings IMHO. But a couple of pics may help with finding of a better solution.