Unlocking domains can be a challenge from figuring out the pattern to the elemental monuments to stepping on panels. Here’s how to unlock the Suigetsu Pool Domain next to the Hydro Hypostasis.
First, pay your respects to the statue. The statue will summon three more Thunder spheres above the pool which you can use to go back and forth between the probes.
Then there are four Lightning Strike Probes surrounding the domain entrance. There are two of the probes shown above and the other two are next to the statue on the pillars. Grab an electrogram from the small Sakura Bough next to the circled lighting strike probe on the left. Afterward, the electro monuments surrounding the domain pool will unlock themselves as shown above. You can use an electro character like Fischl to strike the four electro monuments in any order and that will partially drain the pool. Then jump down into the pool and swim through the narrow hole, and you’ll see stairs when reaching the end of the path.
Go up the stairs and climb the wall to get to the device above the hydro slimes that will drain the rest of the water away as shown below.
After that, you’ll be able to access the domain, the time challenge, and the relay stones for the stone puzzle above.