The tank in a water heater is a pressure vessel and a fairly substantial piece of steel or fiber reinforced plastic. An old water heater that isn't leaking, decommissioned for other reasons, makes a good water storage tank. If you route you water through it you will always have an extra 20 to 40 gallons of fresh water on hand.

If it is a steel tank it can make a nice grill. One issue in this is a steel tank will have glass fused to the inside. This can make working the steel more hazardous, wear safety glasses, and slightly more difficult. Of curse a plastic tank is useless as a grill. Then again a plastic tank is far less likely to leak and makes a better holding and/or pressure tank. I have seen a new one used in a solar water heating system because the water heater was less expensive than the holding tank.

The electric ones usually have deeply dished bottoms.
If you cut the bottoms off they make fairly good watering bowls and stuff like that, if you cut the tank lengthwise they make bigger waterers or feed troughs.
The steel tanks are thick enough to make fairly good stoves with.