I've often wanted to explore the cave river systems a little, or cross a lake, and I've had this little idea: boats!
The way I envision it would be like this:
1. You build a dock next to water (it must have 3 tiles over water, similar in nature to magma buildings) to hold your ships.
2. You build a boat workshop.
3. You make a task to build a boat.
4. Once the boat is made, the Dwarves haul it to the nearest dock.
5. You can give the dock tasks, like a workshop, but the tasks consist of this:
(e)mbark on an expedition.
(f)ish the surrounding rivers.
e(x)plore the surrounding area.
(a)ttempt a landing at another area of land.
The first option brings you to an embark screen, where you can choose which Dwarves will go (preferably those with high strength and swim skills) and what provisions to take - then, you can choose to set off upstream or downstream. The Dwarves will try to keep the boat in the middle of the river (based on their shipdwarfship skill, trained by using boats on still waters) and try to reach an edge tile. When they do, it will create a new fortress in the next tile along, and then you can save the current game and load that one, and you can start up another fortress next to the underground river.
The second option has the boat filled with fisherdwarves and they sort of move about a bit while fishing. It increases the rate of fishing if you have them make a *rope reed net* at the boat workshop.
The explore option just lets you pick patrol points for the boat (like a squad) and has it move around, so you can uncover new veins of gold or new places to explore.
The landing option allows you to choose a spot to land the boat and who goes on the boat - so you can set up ambushes for invading forces by coming out of the water behind them.
I imagine a boat as looking something like this:
<====>o
With the circle at one end being a figurehead (think statues, for their description). I imagine it as being a longboat, as that seems more dwarvenly than other kinds. They are based on Norse mythology after all.
Of course, the boat has its fair share of horrific deaths fun, as poor skill in a fast river could easily mean the boat smacking the rock and breaking, and dwarves plunged into the rushing current of carp. Talking of which, they'd be liable to be attacked by all manner of aquatic life. What fun!
Perhaps obsidian boats could be used to travel the magma rivers? Who knows?