Lucumo: How to open that one door? Was randomly exploring the world and stumbled over it. Beat that monster thing in the North but can't open the door a bit back.

Also, is the game that easy or am I overleveled (was level 15 when I beat that monster) for this? It's not like I killed monsters on the side, have only followed the quests so far (until I saw the path down there while escorting the head).

PS: Will I ever be able to swim? In the temple there are some ladders and ways but they are inaccessible because of the water.