Why does nobody create an electric board that is waterproof with grippy wheels? I know it's possible if waterproof cameras and waterproof electric motors exist.

grippy wheels are tricky. You can't use urethane, but instead need to use rubber, which is a lot less efficient (heavier and slows you down too much as it takes too much effort to push with the deformation/heat loss). What you can do is buy grippier wheels and swap them onto your board (get maybe a 78 hardness compound) and get wider or bigger wheels too. Easy to swap out. Regarding waterproof batteries, I agree entirely.

(Assuming you are familiar with this climate) Would you say that Seattle is out?

that rules out San Francisco northwards, then, and certainly much of Western Europe, surely?

Even in San Francisco, we can't use it everyday. We have ridden it in Seattle and thoroughly enjoyed it.