When do they break?
Do they shatter when you are putting them in the water, or do the break when you try to move them on the model?
If the former, then the Microscale (or Testors)decal film should help.
If the latter, then a decal setting solution (like Microset or Testors decal set solution) should help. In a pinch, you could also use white vinegar for this. Apply some to the model where the decal goes. Set the decal on the puddle and adjust its position as necessary. Once it is in the proper postion, blot with a soft tissue or q-tip to remove access liquid.
When you are applying the decal to the model, handle it very gently. Try to only use a soft paint brush or q-tip to move the decal. If it gets stuck, apply more setting solution.
Do not confuse decal setting solution with decal solvent! You use solvent only once the decal is in position, to melt the decal into the details of the model surface .