Sea sickness can be fatal, mind you you have to be really bad. This is always a concern for the captain who should of course be aware of any one feeling the slightest queezy. You wouldnt send a man forward to fight the spinaker if he felt a bit off. In rough weather it is the captains responsbility to keep an eye on any one sitting quitely or going below constantly.
However I have heard that the last resort to serious sea sickenss is to throw them over the side. No I am not joking --apparantly this works. Of course they are tied and the conditions are suitable. Or you lower them into a dinghy.This is supposed to eliminate to second motion of the boat, so the "patient" is actually in contact with the water. It also stops the puke from destroying the teak deck.!!!! This apparantly will lessens the confusuion between the information coming from the eye and that from the balance information of the ear. Its this confusion that will make you sick. Going below is not the thing to do as the confusion is then at its worst, eyes saying nope nothing is moving but the balance say whooa. It is always recommended to sit and look forward at the waves, then when the wave moves the boat you are expecting it and cuts the confusion.
Its always best to ask first before throwing some one over the side, If they say "yes I want to die",-- they are OK
My wife got sea sick sleeping in a water bed, It wasnt long after that that we started to drift apart.