If the bike is made properly water won't hurt anything. Smearing a little automotive dielectric grease on all the connectors is a good thing to do.
Have to disagree with the statement a shop is needed for the motor and electronics. If you can maintain your own bikes than you should have no trouble with the motor and drive parts. Lots of info on line.
Also these bikes use software to enact the 250 watts and 20 mph limits. The Bafang units can be reprogrammed to vary the limits. I've cruised at 38 mph on my bike. Not sure that is possible with the Bosch stuff.