There are top quality sea sickness pills and patches and such that can be had over the counter or via a prescription. I'd get those and not worry about it.
As I've said many times, go to any in-town pharmacy and buy some cheap Cinarizina (MX) pills known as Cinnarizine (Elsewhere). Available in MX and Canada and elsewhere but not the USA for some reason. It's the sea sickness pill of the Royal Navy so if it's good enough for them it's good enough for us. We get a bottle of 75mg pills and cut them in 1/2 and keep them for several years. We'll take a 1/2 pill about an hour before diving if it looks really rough. No sleepiness, no problems and it's dirt cheap in MX. But the various patches that cost much more by prescription in the states are highly effective to and could be even better but I can't say as I've never been seasick and never needed a patch or something.
I've also noticed that people tend to get seasick on the slower, larger cattle boats with far more frequency than the smaller and faster 6-10 packs. It seems counter intuitive but it's true. For some reason those bigger boats that tend to pitch and roll more combined with the stench of diesel exhaust in the air make some people sick quickly. On a small boat you can see all around, know what's coming, no stinky exhaust getting pulled back into the back of the boat and see the horizon all around.
Sea Sickness is the result of the fluid in your inner ears that gives you balance being out of whack with what you are seeing so if you're on a bigger boat looking around everything seems relatively stationary and you can't see all around so what you see doesn't match the signals your inner ear is sending your brain causing a conflict that causes seasickness. This is why the absolute worst place one can go if they feel sick is into a cabin on a boat. Getting to the front of the running boat with face into the wind, see what's coming and keeping your eyes on the horizon is much better.
Was on a cruise ship years ago in some swelled up seas with the ship gently pitching pretty far to port and then to starboard and back and people were seasick everywhere. On a ship that size if you're in the interior anywhere it's like the whole little world you see isn't changing but the whole little world is pitching back and forth. That's a recipe for disaster for some people.
If someone is highly prone to seasickness, I wouldn't get on a larger cattle boat without a prescription patch or Cinarizina and would stick with the smaller, faster 6-10 packs.