To my knowledge there is no such thing as a waterproof e bike. Just don't use water under pressure around any of the electronics on the bike. Use a brush to get most of the crap off and then water it down. Best thing to do is to keep it in a warm place after washing to help evaporate moisture build up. Water resistance is the best one can hope for and that means shielding the connections, controls and motor from intrusion above and beyond what the manufacturer has done to the best of your abilities.
One thing I find useful is to take a section of inner tube, slit it, and install around connections using a zip tie to hold the top to the cable. Don't close the bottom as you want any moisture that may happen to get in there to be able drain away. Need to get the connector apart, just peel the tube section back. Ugly but effective.