Medicate early and preemptively. That one decision matters more than everything else combined. Once the weather has turned and you already feel unwell, you've missed the window and you're managing rather than preventing. Take whatever you're using before you leave the Beagle Channel, not when the ship starts moving.
What travellers most commonly use is scopolamine patches, though Dramamine, ginger and relief bands all come up. Patches shouldn't be mixed with alcohol and they come off in port. We're not medical professionals and neither are the expedition guides, so talk to your own doctor about what's right for you rather than taking our word or a forum's.
Every expedition ship carries a doctor who can prescribe and help. Sick bags appear around the ship before a crossing, food can be brought to your cabin, and drinking plenty of water helps.
Cabin choice makes a difference too. Lower and mid-ship feel less motion. Avoid the bow.
The last thing, and the one people need to hear most: put your ego away. This is a once in a lifetime trip and there's nothing admirable about being miserable for two days to prove a point.