DISCLAIMER: most of the stuff about the temple’s history is speculation from scarce sources.
This ruined temple rests 1.25 days of travel to the north east of White Moon Cove, right on the cliffside. The temple was originally built by unknown people and was dedicated to the aquatic goddess Umberlee, Queen of the Deep, who represents the waters’ more destructive side – sailors pray to her so she doesn’t capsize their ships. Caution is recommended (although that stands true for almost any place on this island).
Inside the temple, to the left of the entrance, is a courtyard where a giant orange tree stands. If perceived trough means of detecting magic, lines of blue unknown magic can be seen flowing trough it. The tree bears fruit that lets a creature that eats it to breathe underwater if it is a land dweller or vice versa. Amphibious creatures seem to be unaffected, though this isn’t fully tested. USE THIS WITH THE UTTERMOST CAUTION! The fruit lets your breathe only underwater or air and we have no idea for how long this effect lasts. I’d love it if you could test weather the effects of one fruit can be reversed by another.
Additionally, once plucked, the fruits’ magic starts slowly fading. I approximate that the magic remains for about three days, but I could be wrong.
The temple has since been abandoned and its inhabitants have been replaced by angry aquatic creatures. As far as I know, they have no connection to Umberlee and simply use the tree to be able to live above water. If you enter the temple trough the main entrance, not the ginormous hole in the wall, you will enter an altar sort of area. On the altar lays a stone coffin that leads to what seems a water tunnel. We saw the aquatic creatures come out of there. If you want to continue exploring the temple, I suggest starting with that place.
P.S. Be safe, Plink.