If you have the most common Water heater, it does not send power to the heater, it controls power from the heater.
You don't say if there is one or two fuses feeding the appliances. If one fuse serves both units, then there is either a break in the power line between the fuse and appliances, or in a common ground for the appliances.
If there are fuses for each device, then most likely the problem is in a common ground.
A check for a common ground problem is to check between the +12 VDC into the appliance, and to chassis ground. If you have the +12, but not between the inputs, then the problem is a grounding issue; if not, then probably a power line issue.
Doug
