Then I realized the 'Mer' in 'mermaid'. And I had a tiny mini brainwave.
Could it have been possible for some Mer in TES to anger a God or Daedra, who would have cursed them to never walk on land again? I suppose it would be very easy to add to the lore if it isn't there. Elves and pissed-off Gods generally offer... interesting results. (Orcs/ Dunmer)
So, could it be possible? What if a group of Elves acted against a greater power, and were cursed for their actions.
Is there a God that rules over the sea? There seems to be a God for everything. What if they acted against him/her, and were cursed to become their servants for eternity?
Just a thought.