My wife used to get sea sick looking at a boat. Really, throw up til the green stuff, slightest motion would set her off.
She contracted a viral inner ear infection that made her sea sick lying on the floor at home. Took a week before she could even get up. The Doc recommended pushing it as hard as possible - apparently the virus knocks out the nerves from the cochlea to the brain and pushing it hard rebuilds the pathways.
She forced herself to get up, vertigo hitting her constantly. Vacuumed the house! Refused to be laid low by it. Threw up constantly.
Upshot is, she is now immune to sea sickness. Totally. I'm reasonably ok with going below in a blow. She goes without hesitation. We did a race
on a tri a few months ago, beating into 15ft waves, short, up and down like a fairground ride. Turned into the vomit comet. Half the crew laid out, decidedly green around the gills. My wife was down there tending to them all bless her.
Not that I'm advocating catching an inner ear infection. But anecdotal evidence that it is possible to retrain the brain.