Tarn has expressed in a quite old interview that he was excited about boats in DF, but doesn't know how to implement them quite well. I suggest that, instead of bringing the large sloops and brigantines and frigates already, DF could start with simple boats that work in a similar fashion to wagons. So most of those propositions are mostly going to consists of small embarcations to begin with. I know that what could be the most complicated aspect of boats (and large vehicles in general) is rotation. So I can assure you that everything I'm proposing could fit on a 3x3 space.
- Raft; the simplest of boats, if you can even call this a boat. Can hold up to 5 individuals and 3 animals, as well as some objects helded by its occupants.Requires logs and ropes to be constructed
- Dinghy; an actual boat, smaller, but has less chances of sinking and break, this 'ship' can hold 3 individuals, 2 animals and/or two objects. Needs the use of two oars, manipulated by a single dwarf/elf/human/goblin/kobold/etc.
- Small boats; bigger than his miniscule counter, this boat can hold twice as much individuals. Must be manipulated by two dwarves/other with each an oar.
- Canoe/Kayak/Pirogue; a long, shaky boat that can be manipulated by 3 dwarves/other using one oar for each. Mostly used for travelling and liasons/messengers. Only practical for longing rivers.
- Small Sail boat; small compact boats used on large streams. With only a single sail. Their fabrication require silk for the sails, and ropes to hold them. Their capacity are considerable, can transport 5 to 7 animals, 15 individuals and various objects. Are less likely to have an ornament on their keel.
- Longboats; mostly used for military purposes, those longboats would be the largest and the most fearsome. Could bear up to 20 men. Similar to Viking Drakars, they could possess a figurehead and ornamental shields. They only have a single mast and a large sail with a motif. Could be used by goblin invaders or enemy humans/elves/dwarves, even bandits! (Arr! Pirates!). Dwarves who live on Islands, beware!
So of course, the long boat would probably be something like 7x3 or something, but you get the idea. I'll let you image the size, but really, consider them to be acting in a similar fashion to wagons.
Now of course, you're wondering how could you build those ships and where to find them. But mostly why would we have such boats and not bigger, meaner, more practical ones.
So first off, to fabricate those ships, you'll need to designate a zone as a shipyard, until then, it will all up to the Carpenter and the Craftdwarves to fabricate each component; the keel, the main mast, the sail, the figurehead, the rowing, etc. Selecting the ship yard, you will be offered a list of ships that could be fabricated. Select the one you wish for and all of your carpenters will build it under 5 to 10 minutes. Once done, all surrounding dwarves will put the ship at sea and attach it to a few or a single support for it to not be taken by the ocean. I think the only practical ones on these list will be longships and small boats/dinghies, for military/liasons.
If you live by a shore or an island, merchants could come by boat and unload their goods nearby an assigned port. Likely, those merchants could only be humans or elves. Dwarves could have more chances to be 'sea-sick' (I know this doesn't really make for a good argument for this suggestion, but stick with me). Only Hill/Island dwarves could be using ships for trading and invading. If you live on an island, migrants could come via dinghies or small sail boats, along with Merchants.
And of course, our precious goblins could come invade Island Dwarves with the use of longships, viking style. Using a siege weapon to sink the ship could be an option. You could assign the ship to be destroyed and it's materials to be reused. Pirates of various civilizations may also attack you at any time. Hope you like Pirates of the Caribbean, cuz it may be possible that you'll reannact that famous port attack scene from the movie/ride.
In adventure mode, you could become the captain of a longship and travel the ocean. In fortress mode, you could assign a captain to a ship and prepare a journey. So you have to think of material, weapons, food supplies and stock goods for your travelers. That is if anyone in your fortress isn't sea sick.
Another thing is that ships could be the subjects of various sea hazards; sea monsters, merpersons, storms, ghost ships, and enemy longships. In adventure mode, it could possible that you find shipwrecks. They may have some goods on them.
These ships wouldn't really rotate, but rather move like wagons, and slide from square to square when moving in a diagonal axis.
Easters eggs;
Elves are more likely to be allied with merpersons.
A messenger/liason from another fortress may comment on how hard it was for him to reach your island; "Gr-Greetings from th-the mountains h-homes. Erk, I'm-m sorry, I'm kind of sea sick and the v-voyage was quite... Rough. A-anyway, we have much to discuss." or "Erk, couldn't you have settled your fortress near a m-mountain? That would h-have been more... Practical."
Human sailors will start to speak a strange language; "Ahoy there, meadows. We have much to discuss, the sea, the landlubbers out there and some terrifying sea wolves."
Elves will comment on their forrest counterparts; "The sea calls, let's make this trade quick.", "May [God of the Sea] bless your fortress, our ships shall acost your port soon enough once more.", "May you one day embrace the waves as much as you embrace the soil and mud.", "By the holiest of mermaids, this is an outtrage!"
Goblin invasions messages could go such as; "Terror at sea! An vile force of evil as sailed to your island!"
Pirate attacks could go like; "The black flag! The captain Urist McScallywag has arrived with his terrifying crew!