My GF and I built a fortress on the sea. The most successful fortress we've ever had with the most elaborate dwarven plumbing we've ever built. Hopefully the images will work for everyone.
The entrance is on the north, one level up from the ground. There are 4 retracting bridges. In theory, even if heavy creatures come across, the small bridges might be able to retract still keeping the creature stuck on the 2 larger middle bridges. In practice all bridges retract before the goblin siege arrives and they are forced to walk the twisty path to the west of the bridges...while marksdwarves fire at them from behind fortifications, when they dodge, they dodge off the path and fall 8 z levels. To the south are windmills that power the desalination pumps.
To the south are the desalination pumps. North-west the hospital. North-east the training rooms. The central-ish pit is surrounded by the stair-way that goes all the way down to the magma forges. The pit provides fresh air and some sunlight. The other pit to the north-east provides sunlight to the baths.
To the north is the bottom of the pit where many goblins have met their end. In the middle is part of the stair surrounding the ventilation shaft. To the south is a 3x3pipe that supplies the fortress with clean water.
This is the upper level of the dining-hall and to the west the kings throne-room. North-west is the other noble rooms. In the dining-hall there are decorated support pillars that each have waterfalls rushing down the sides.
This is the lower dining hall with wells around the perimeter.
Two z-levels below the dining-hall is the cistern, the z-level in between is the overflow path which is routed into another large waterfall at the baths. With the cistern in place even if the pumps stop working or there is an unnaturally cold winter that freezes things the dwarves still have enough stored water to last the season.
Here are the baths. There is an overflow for them in the south wall (though one z-level below this) at this point the water acquires contaminants and the overflow from here won't be used as a drinking source.
The tombs with a mousoleum to the east. The king's tomb is to the south and the 4 waterfalls there (fed from the bath overflow) are the final water feature of the fort before it's all routed off the map with a tunnel to the edge and a fortification.