I think you are fine on the boat as long as it isn't too choppy and he goes at a nice and slow speed.
This. I live on a lake in the Adirondacks (upstate New York) and we use a boat (we have a pontoon and a motor boat) during the season's which allow it as a mode of daily transportation. 95% of the time I wouldn't have a problem, but we have been in a situation when we get caught in a storm in the motor boat and get bounced around like nobody's business. I am like, literally airborne, bouncing up and down on the seat from the rough water. So PERSONALLY I will be checking the radar before I find myself in a situation like that while pg. Have fun on your boat with your man and maybe just keep an eye out for any stormy weather :-) There is nothing like those lazy days on the water, what a great way to enjoy your final months of being just husband and wifey.