Water under your water heater could indicate a couple of different problems. The first step is to look for a hole in the external tank or pipes. If you notice that your water is running constantly, your water heater is probably leaking and must be replaced by a professional. If you notice water coming up from the floor drain when you use other plumbing in the house, then there is probably some type of blockage at the junction of the branch line and floor drain. If this is the case, you should call a drain technician to clear the line.
It a good idea to place a metal drain pan under the water heater, especially if it is located in the attic or a small interior closet. This can protect your flooring in the event of a leak or tank failure.