In general it is, if the wire ends are sealed as I said they should be.
Hero while you're right so is Externet, a speaker in a sealed PVC pipe would waste most of it's energy as the energy reflected off the PVC pipe many times, basically ending up as minuscule heating in the plastic, some sound would transfer through the PVC pipe but you'd need a flexible membrane in direct contact with the water moving to transfer sound efficiently. So it really goes to asking what you consider an under water speaker, one that works, or one that works well =-)