One thing we tried to do with our divers, if all else fails, and you know you won't be diving that day is to get into your wet suit, with no gear or weights on and hang on the Gerry line while the other guys are doing their dives. This may sound crazy, and many of sick divers won't even think of this as an alternative, but once you're off the boat, all the rocking and rolling goes away. You just rise and fall with the swells, and the motion is 1/10 of what you are getting on the boat. Stay back away from the other divers. Experienced guys know what's going on when they see you, so don't think they will be bothered. As I say, just stay back a little way, wear a PFD, and take a small line to tie your wrist to the line. Tell the DM you are doing this, and they will keep their eye on you.
Okay, I have to drop the BS flag on this.
For some of us, it is the elevator motion that works against us, the up-and-down on the waves. I am just as sick ON the water, sometimes worse. It lasts to about 10fsw. I dread coming up from my safety stop.
I must note that I've run into only two people that are affected as badly as me, and they, too, relate that it is the up-and-down, and not the rock-and-roll, that get to them.