Dan, there are straps made specifically to anchor water heaters not located
directly aganst a wall. In fact, most of the installations I've inspected
did not bear directly on the wall in back of the heater. In seismic D1 & 2,
you only need to strap in upper and lower 1/3 to withstand movement, and
there are many ways to accomplish this.

well-installed. To put in a tank water heater would be problematical
because it would have to stand away from the wall at least enough to
clear the dryer vent. The contractor who authorized all this told me I
couldn't do that because I would not be able to strap and support the
tank. This is earthquake country.