I'll mention one other thing nobody else has...something that triggers nausea in a lot of people...diesel exhaust. Some boats are more prone to build up diesel exhaust, especially in the main deck area if the "upper deck" covers a large portion of the lower. The next time you go out on this boat, try the following:
Step #1, get to the boat early enough to get all of your gear assembled, tested, and your extra stuff (gear bag, shoes, etc) put away BEFORE the engines are started. Sometimes this isn't easy due to the Captain's particular pre-trip routines, but most boats don't sit and idle for long before departure thanks to the high cost of diesel.
Step #2, as soon as step #1 is complete, go up onto the upper deck, or bow of the boat. Take your wetsuit with you. Use it as a pillow until it's time to suit up. If you have to go back to the rear deck for a safety briefing, dive briefing, etc, only go as far as you have to go in order to see and hear the briefing. (in other words, stay in the fresh air)
Step #3, When you hear the "gear up" call, put on your wetsuit. Don't fully zip up or put on the hood if that's what you're diving. This is the key part for many people. Ordinarily at this point you are under the deck, totally zipped, wrestling into your gear while the boat is being maneuvered into place. This is often when you take on the most diesel fumes, especially if the boat is hitting reverse when looking around for their anchor point. (If it is to be a "live drop," meaning no anchor, just wait as long as you can before going to the main deck.)
Step 4: As soon as it's clear that people are about to hit the water, zip up/hood up and go onto the main deck, (turn your cylinder back on), slip into your gear and move as directed to the entry point.
Step 5: Enjoy your dive, nausea free!
Hints: Don't be shy about communicating to the crew that you need to stay in the fresh air. We really don't mind people hanging about if they are trying to avoid getting sice (read as "trying to keep to crew from cleaning up a mess). Also, best to have a chat with your buddy we enough in advance that they aren't getting concerned if you aren't geared up 30 seconds after the "gear up" call.