thank you gerry bp. that works, and i agree, i have a wahl 9818L. experienced the exact issue. will not turn off after shaving with water. i removed the trianglular ON/OFF pad gently, ( if you remove on side and gently lift it up, you will not break the plastic clear switch away), then wipe the inside with a q-tip. press the on/off switch with a firm pointed object. it should turn off and then repeat for on.
i didn’t want to seal the switch but, i might have to place a breathable water resistant cover over the switch to prevent this from re-occuring. oh, i meant over the top of - not under the switch.