I found it pretty easy to start (in Java, with JME3) with a simple sea that was just two sinusoids at angles to each other.
My first floatation was to give my ships four invisible outrigger pontoons that stayed on the surface, and have the ship follow them.
I later moved to having a physically-based one that took the hull model's faces, calculated their centroid's depth, and then the force of the water pressure being exerted through that centroid. It's a good approximation, I'd think, if your hull model is fairly detailed (I used a lower-poly version for this than the one I'd show visually). I never got that one fully under way.