In the old days when I was more up to date with this sort of thing the thermostats did not always switch the load directly but used an intermediate relay.
K.I.S.S. ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D
As Brian appropriately points out, a good approach for X10 control of water heaters is to use a suitably high current contactor, controlled by a wire-in appliance module, all tucked into a suitable electrical enclosure...
And, as Oldtimer succinctly pointed out, that " additional expense " may not be necessary...
;)
K.I.S.S. ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D