Missable! Find this tablet as soon as possible! You can pick this one up after getting the ice skates for Knaus (the little boy with the green shirt) and meeting the other three guys at the village of the old lady.
The tablet is located under a small island at the bottom of the giant lake in the middle of the map. Teleport to the totem near the crashed airship and ice skate to the south of the lake. Go down the ladder and pick up the tablet.
It's important that you do this before lowering the water of the temple.