I spent a lot of time on Leeuwin sailing as a kid and young adult. When we went out on a ten day voyage (My first was the Bicentennial voyage Melbourne to Tassy ) we were all told about these miraculous pills that would stop us being sick with in the hour. But as they were expensive we had to tough it out first few days and most would just get over it naturally once they found their sea legs. Cool.
So next trip out and i sailed as a watch leader and we just taught what we learnt as trainees. We also had to watch the people who were very sick and report their condition regularly to permeant crew. Anyway this one girl was in a very bad way after three days and they agreed to prescribe one of the "Miracle" pills. Well first they scrapped the letter G of it and i was forced to give this poor girl (Who i had lied too) a placebo

Anyway literally almost to the tee an hour latter she was sitting up and asking for food. Needless to say, i threw up for the next day after learning id been tricked all those years.
Apparently two different types of sea sickness. First is simply in your head, sell a placebo well enough, it will work, second is a real condition that often is associated with travel sickness, dizziness and even inner ear and sight problems. These people can normally find medication that will work. But it can take different methods for different people. Some will have them the night before and then the morning also, others might need them on food or others on empty stomach. Their are a lot of different medications out their, and just because the first one doesn't work, don't give up..
For people who get sick i find also good communication and explanation to them as to what is happening on the boat and voyage. Often nervousness and anticipation are the biggest problem. Take their mind of it and they'll be much better, obvious i know, but i find communication is such a key for happy boating. What we might think is obvious can often be very foreign for others.