A jacket for a water tank comprises a sheet of water-containing material such as plastic. A closure is provided to draw together two opposed edges of the sheet. A fastener, such as a strap clamp extends along a third edge of the sheet. The jacket may be retrofitted to an existing tank, such as a domestic hot water tank, by clamping the fastener around a drain pan under the tank and closing the closure so that the jacket wraps around the tank. Any water leaking from the tank is caught by the jacket and directed into the drain pan from where it can escape safely into a drain.