Another vote for Stugeron here. My problem isn't the rocking of the boat it's the smell of the diesel - a good lungful of that and I'll honk for England (if you'll excuse the venacular) but I find the Stugeron helps prevent me from actually being sick.
I managed to get through an entire day in Anglesey on Scott's boat in Anglesey though the last time without anything
Ginger is really good, especially the crystalised stuff, and I find that avoiding some of the richer/spicier Egyptian lunch items to be an advantage too.
HTH,
Noel
Added: Oh, and tell them to drink plenty of water.