Just to give some context here so you won’t be running like crazy trying to 100% before even being able to.
This will probably be your last region to 100%. The reason is because you need all 52 bloodstones (meaning, all other regions will be 100%, unless you forgot a teleport pad or chest)
With the reward from having 52 bloodstones, you can open a special chest with Warlock key.
Arne village has a underwater entrance to the lighthouse as well.