First, the biggest problem for drowning in a car isn't seatbelts but windows. People will remember to unhook the seatbelts, and by that time it will be too late

First, the biggest problem for drowning in a car isn't seatbelts but windows. People will remember to unhook the seatbelts, and by that time it will be too late to open windows. In essence, you're arguing for the removal of windows, not seatbelts. Second, the nirvana fallacy is comparing actual things to unrealistic, idealized alternatives. You're comparing actual (seat belts) to an idealized alterative (something that prevents all car crashes, no matter the cause). Third, it's a less generalized example. Stuff seatbelts don't protect and might even lessen chances of survival - rockets, gunfire, rhinoceros, trains, meteors, nuclear blasts, black holes, multiversal collapse.