If the molasses has been mixed properly and the urea is completely dissolved there is no chance of the urea dissolving in the water. The safest thing to do is leave the water sitting on top of the molasses. The water will either slowly evaporate or the cattle can safely drink the water off the top to reach the molasses. The remaining molasses is safe for the cattle to consume as water is lighter than molasses and therefore will only sit on top of the molasses and not mix through.
NEVER stir the water into the molasses as you run the risk of diluting the urea concentration to dangerous levels, increasing the risk of urea toxicity. Mixing the water into the molasses can also cause fermentation problems.