Light Under Sea is located in The Loop region, unlocked after sending The Voyage of Discovery voyages (15,000 Morale, 15,000 Combat, 15,000 Seafaring) that unlocks it. A faint actinic glow under inky waters is the only sign that wisps inhabit the coral caves honeycombing the stem of this tiny island.
When the voyage presents itself: "Rumours have reached your ears regarding an unexplored island known as Light Under Sea. The opportunity beckons to add a chance of attracting a wisp to your crew."
Successful completion of the voyage unlocks the ability to hire the wisp to your team. The wisp's base stats are: Morale 300, Combat 750, Seafaring 750, Speed 800, and its trait is Rallying Cry (10% more XP on any voyage for captains and crew on the same boat as the wisp).