Yeah, on the Rivian, in rain, typically I don't get any water intrusion in the bed, but I get some drops in there after a brushless car wash. But that never bothered me since I only store surfing gear that can get wet, or is already wet. And for things that I need to be dry, I have the massive frunk and gear tunnel, under seat storage.
I see the worry with Cybertruck since there is less frunk space, but if I'm not mistaken, isn't there a lot of storage space under the rear bench seat? Maybe even similar to the gear tunnel?