Has a little time this morning to go webdigging.
According to
this Wikipedia article Plymouth locomotives were mostly mechanical or hydraulic drive. So as long as the water wasn't high enough to flood out the engine and controls it could wade like the steamers (with a quick service of the drive chains afterward I'm sure). Hydraulic drive is confirmed for some later models used in Vietnam in
this Wikipedia article.
GE 70-tonners might be a little more dicey. Electrics in general don't like to get wet about the motors...