Hands down, sailing. It's so well done, and so very engaging. You feel so involved in it, and there is definitely a learning curve to it. Each boat serves it's own purpose;

Karves are good for exploring and discovering new land, taking rivers to the heart of landmasses, and making even the tightest turns in the smallest of cove.

Longships are good for long cargo hauls, whether transporting silver or iron, and they can brave the fiercest of storms and conquer the mightiest waves of the ever-churning ocean. In a Longship one has such confidence to hunt dozens of serpents, and the hunter is in command of a heavy vessel of beautifully carven wood, worthy of legend.

And rafts, well, erm. They work well as small ferries! Maybe for rowing a small distance? Plop one in a river as a way across?

And the wind, oh the beautiful wind, you need to pay attention to it, and work with it, not against it. There have been several times when I've had impromptu expeditions because the wind was blowing the right direction one day. It's so satisfying to unfurl your sail, let it go on a beam reach, and off you go, off to navigate the seas!