@toxman
My first dive from a 40 ft boat in 6ft seas was a failure because of sea sickness, and this was my first time. The first problem was that the ocean was way too rough. Only go in calmer conditions with good visibility. Don't fiddle with your equipment at the last minute whether on a boat or not. Don't get hot. Be well hydrated
And be relaxed. Anxiety over the dive will make sea sickness worse. My second trip was successful but I still felt sick under water the first dive and it was because I was so nervous. By dive 4 the same day, my air consumption was halved, I felt calm, and the nausea disappeared.