For the rainy season around here (FL,) I got a cheap set of bearings and put some lithium grease in them to keep the water out. The grease will eventually wear the bearings down but it definitely keeps them from rusting, and they actually get a bit of vibration dampening out of it, which is nice on our gravelly-textured roads.

Stopping distance would be the real problem for an electric board in the rain, though.