I have a few that did this, thankfully it only caused some corrosion on the battery terminals and was an easy fix, if that's the case with yours you can check out my
work shop.
One of the problems I find is that the O-ring that most toys use is damaged or too small to function properly. Also, making sure the battery lid/cap is tight can be an issue too; we tend to worry about tightening it to much and breaking it. One way to fix this is to head to a hardware store and buy a O-ring that is slightly thicker. This may or may not fix the problem but it only costs about 25-50 cents so no big loss there.
Unless there are lots of reviews stating that it is indeed waterproof, I only consider them splash proof at best.