Nope. Damp rag, rinsed often in a bucket of water. Single trails are closed after rain, so it never gets really muddy. Probably looks odd, but I carry the bike over a couple of streams out there too. My theory, water and electricity don't mix.
Regarding haibikes with a bosch motor, those things are nearly waterproof.
Its obvious that you shouldnt spray that thing down with a pressure washer, but as far as I know, there should be no problem rinsing the bike off with a hose (without pressure ofc).
Battery off the bike, there's no chance to short the system even if a drop or two of water get's into a connection.
Battery mounted, the system will be looking for that opportunity.
I live in the arid west and a bucket wash is the right way to do it with the battery off the bike.