turbulence
Belted should prevent being one of those on TV news who hits head on ceiling. Boats experience "rogue waves" on rare occasion...maybe analogous to clear-air turbulence?
OP: turbulence doesn't seem to cause many plane crashes. Seems to injure a few of the unbelted during flight phases when belting isn't mandatory. I prefer flight turbulence over the slow rocking motion of large boats, which can induce sea-sickness.