All depends on what activity you intend doing.....?
Just sat at the helm of your vessel, or a little more arduous activity!
I have a one piece mullion floatation suit, that I kept from my days working in the boat yard. Whilst working in it, I just used to wear shorts and t-shirt under it, even in the depths of winter,as it was very warm. If I am using it now, on my boat, I just wear jogging bottoms and a sweatshirt. I also used to use it whilst on my motorbike, for the commute to the boatyard..!
Fantastic bit of kit.