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During world generation, seas and oceans will fill the lowlands (and lakes the bottled-up highlands) of your map. Islands are often volcanic, but can also simply be the result of mountain ranges that are mostly submerged.
Starting your fortress on an island
Islands vary in size, but even the smallest tend to be very large relative to the size of a typical fortress. In bodies of saltwater, islands tend to have little exposed stone and almost always have saline aquifer layers from sea to shining sea, all making basic survival difficult. Islands in freshwater lakes pose fewer complications, although they are more rare.
In either case, civilizations are rarely founded on islands. Generally, only your founding civilization will brave the seas to either trade or harass you. On the other hand, megabeasts may appear on your island.
Should you choose to build your mighty fortress on an island, make sure you know how to turn salt water into fresh water.
It is likely that, on an island, you will not receive any raids or ambushes whatsoever. This means you will only have to protect yourself from (mega)beasts, and the occasional berserk dwarf.
Evil Ocean vs Good Ocean
The total lack of ambushes and sieges can be boring for some players, in which case choosing to play in an evil ocean might be a nice addition. This will cause all kinds of horrors from the depths to attack your fortress, including the horrifying zombie-whale.