Promethazine is what the shuttle astronauts use and space sickness is said to be far worse than sea or air sickness. It is reportedly very effective and in terms of drowsiness they have found that people on promethazine are less drowsy and more able to perform tasks than those suffering from space sickness.
I am pinning my hopes on this stuff for a forthcoming boat journey to the Antarctic. It had better work or I may come back very thin!
QDM