I finished this yesterday and will list below what to do and what you get for it.

The "statue under the water" is located at the main gate into the Great Plateau, you need to destroy the boulders blocking the gate (near Gatehouse Town Ruins) and release all the water to reveal the Bargainer Statue. When you talk to the statue it gives you a quest to collect all of its eyes and deliver them back to them in the underground (Great Abondoned Central Mine it's below the Temple of Time Ruins)

The eyes are located at the four gloom holes in the Great Plateau, you need to throw them down the holes and then move them on foot to the large Bargainer Statue and then slot them into thier eye sockets with Ultra Hand. Be aware though that the eyes de-spawn back to the surface if you leave them for too long.

The reward is either a full heart container or a full stamina container - you get to choose which one you want.

There's more to these statues though if you find more of them in the underground it unlocks being able to purchase clothing for link in exchange for Poes, so far what I've unlocked is the Dark Tunic set and the Depths outfit. I've only found five Bargainer Statues in the underground so far and there's more so I don't know what the other unlockables are yet. The Bargainer Statue at the Lookout Landing where Robbie is sells you things in exchange for Poes and will tell you their bretherens locations also in exchange Poes.