Your first dive from a boat is a different experience from your first dive, and the difference is entirely on the surface. Forty minutes of the day happen on a moving deck and none of it was in the course.
Take the seasickness tablet
Before you leave, not when you start feeling it. By then it is too late and you have an hour of the crossing left. Ginger works for some people; the tablet works for most.
Sit at the back, in the air
Midships and low is the least motion, but the stern gives you horizon and wind. Inside the cabin with a fuel smell is the worst place on the boat.
Kit up sitting down
Everybody stands up on their first boat dive, and everybody falls over. Sit, get into your BCD on the bench, and stand only when you are told to.
Learn the roll before you need it
A backward roll is the most alarming entry the first time and the easiest one afterwards. Chin to chest, hand on your mask, and let the tank take you over.
Your kit lives in one place
On a boat with eight divers, everything looks the same. Put your fins under your bench, your mask in your fins, and never on the deck where somebody will stand on it.
Look at the horizon, not your gauge
Reading anything on a moving deck is the fastest way to be sick. Check your gauges before the crossing starts.