It will just stop boosting the engine. If used properly (all precautions and safety measures explained in the User Manual and DVD) there will be no meltdown, and no cut off switch is necessary. Between several cars and many experiments, I burned one fuse a year and half ago because it was too small (5 Amps). I started using 8 Amps or 15 Amps, and since then no problems of this nature.

In any case, the absence of water will cause electrolysis to cease, and the device will actually cool down and become inactive, without burning a fuse or melting anything.