I went on a day long halibut charter in Alaska during 2nd trimester with DS1. The water was extremely rough and I did vomit once but I think that would of happened pregnant or not. Everyone on that boat was sick. I would go again though especially because we got our limit and had our freezers stocked for almost a year.
My only advice would be sure to bring plenty of food to snack on throughout the day as opposed to eating large meals. It has always helped me to keep something in the stomach at all times while on water.