Here's a few tips for you Paul,
Tea or coffee can be worse than fizzy drinks.... Like Mart said water is the best, if you suffer a lot bring a packet of cream crackers. Eat the crackers dry you will find that a lot of the time you will keep them down, but if not they will stop you empty reaching. They are also easy to eat when not feeling the best
Also bring a large bottle of Coke for the same purpose as the crackers but before you drink it shake the sh1te out of it and get rid of all the gas. Flat Coke is excellent for settling a sick stomach after your throw up or and to stop empty reaching
Most important of all is to try hard not to think about being sick by staying busy fishing, stay out of the wheel house if you feel queasy, don't concentrate too much on anything like reading (even a text msg) or a tangle. I know when most "casualties" are about to throw up, the first symptom is they tend to go quiet and stare at something usually just a spot on the deck or their shoes and in no time they're off so if you find yourself staring at nothing busy yourself by checking your bait or changing tactics etc. Think about winning the lotto instead of getting sick
Always throw up over the side, going into the heads will only make you worse. If you are feeling really rough lay down on the deck / engine box etc for 10-15 minutes looking to the sky, that helps some people.