Currently, there are no more plans for ocean travel other than a single note on the development page. Let’s expand some options:
Boats should be buildable in multiple sizes based off of pre-made templates, and these can be used for trade and travel alike.
You should be able to build docks in fort mode where boats could arrive from.
In adventure mode, you should be able to construct your own boats, which you could travel the seas with, using the Travel screen. If you reach the edge of the map, you should be forced to turn back.
You could be able to encounter friendly and non-friendly creatures along the way, and you can stop in the middle of the ocean if it is necessary to fight said creatures or repair your boat similarly to the Build screen.
Building boats in adventure mode (and fort mode, maybe) should be like making a real boat, and this would require wood and cloth, not to mention time and effort. The construction should generally be like drawing a boat shape (of course dwarf fortress style), and then placing a mast and staircases. There should be at most 2 floors.
When creatures attack your boat, they could come up onto the boat or try to break the lower layers of the boat, letting in water. If a decent amount of the lowest layer is filled with water, maybe 70-90%, your boat should sink and your boat should quickly fall apart and become logs, meaning you would have to swim back, which may be incredibly hard and deadly, considering how well DF people swim and that whatever sank your boat is still there.
If both floors, you could load supplies, like food and fresh water (in barrels), or even trade goods. Food and water should deplete while traveling, if you have companions on the boat, and this should also count yourself.
Trade ships in DF should be like Caravans, but would have more stuff like fish caught on the journey (which would depend on where they travelled), and generally stuff traded from other port cities.
These are all just ideas to consider, and not all of these ideas may make it into the game.