Wandering islands are islands that are not attached to the physical seafloor and may move their locations on the ocean, sea, or lake they reside on. Fictional wandering islands may float because they are made of a giant collection of kelp, flotsam, jetsam, or other floating substance, or they may actually be the backs of some massive, half-submerged, sea beast. There may be shipwreck towns, raft towns or other towns located on the island.
When designing wandering islands, consider first what keeps them afloat, and second if they can be piloted. Wandering islands may move with the sea currents, with the whims of the leviathan they reside on, or may be steered by those with the skill to do so.