Amphibious vehicles are already available, but extremely costly and impractical for daily use. The times to fill the average auto, would depend on several things, open windows, gaps in peddles (linkage), heating or air paraphernalia, trunk structure and so on, but if in deeper water will sink, before completely filled. As suggested the last place to be during a tsunami, might be your car. Those picture/films of the Japanese Tsunami, tossing around all vehicles, trucks, homes and debris is pretty explicit.
Most US Nuclear Reactor containment units are built to withstand large aircraft impacts or major earthquakes and since the Japanese models withstood one of the worst ever magnitude quakes (9.0, 5th highest on record), several times what they were designed for, I believe any US facility could. Of course, if a Nuclear Plant (San Diego/LA) was operational during the worst ever earthquake say 50 miles off shore and a tsunami followed, causing power failures, the theory that will get promoted is, yes the same things could happen here. I don't know if California has a history of the type a quake as was in Japan (one fault going under another), but I don't think so and these are typically the worse...