Water heaters, when full, are especially heavy and they are typically supported in a cantilever fashion, i.e., only the bases are secured, and often with nothing more than tapping screws, on which significant leverage may be applied.
While on an offshore passage some years ago, in a lively seaway, the water heater on the vessel I was aboard broke free of its supports and collided with the engine, holing the water heater’s tank, which discharged 180° F water, and bending a fuel pipe, which cracked and spewed pressurized fuel, into the engine compartment. When I opened the door and looked into this space my jaw dropped, the water heater was retained only by its hoses and electrical connections.
Water heaters must be robustly secured, many manufacturers now require support at both the base flange as well as at the top of the water heater, between it and the overhead for instance.
Steve D’Antonio