Charlie,

It is low pH, not high pH, that accelerates rusting. Significantly elevated chlorine levels can accelerate corrosion, so a pool using chlorine with no CYA in the water (such as many indoor pools) would be at greater risk. If your heat exchanger in your heater is made of copper, then the rusting is most likely accelerated by the higher salt levels in the pool that are required to operate the saltwater chlorine generator (SWCG). When you get a replacement heater, get one that is more chemically resistant such as cupro-nickel or titanium. One can also use a sacrificial anode (usually made of zinc) buried in moist soil and connected to the bonding wire in order to reduce the rate of metal corrosion.

Richard