No matter where you go, you will always get this same argument.
Most manufacturers do clearly state that their mats are wipe clean, so they are waterproof to an extent, but they should not be submersed in water.
Move the water bowl to the cold end, less chance of the water getting tipped directly onto the heat mat. Use a heavier water bowl.
Finally, for a young corn snake, I'd say a heat mat is fine, I fully understand that heavier snakes can cause issues (their urine also tends to be in larger amounts, which could well cause a problem)