The Sun Temple
This is lengthy, just take a seat and read. ALL OF THIS is detailed info about what to do and how you can reach a boss fight more safely.
Once you get into the temple feel free to explore the big room and its halls. Here you will raise and lower water levels to progress through its Mechanical-themed environment and "creatures". You will find a dark area deep within the Temple, but you can't pass without light. There is a way to illuminate its waters, but you must roll near a big gear at the water's surface (you can touch it). This changes the water's depth- it will rise up to the necessary height on roll #1, and revert to the original level on the second roll. This will allow access to a room in upper right corner that has a blue Crystal on a floating wood platform. Above the crystal is a message—written in Aquarian Runes—that says "infuse with light". Singing the bind song lets you drag it around, but it will remain afloat. Explore the opposite side of the Temple to find a Save Crystal and a vertical passage leading to a room where the Sun's rays shine from a window.
The puzzle is simple now: position the Sunstone so that you can move it between rooms. This is done by manipulating the tide by turning the Tide Gear via rolling. You will need to swim fast (between tides) or jump (low tide) to the right to position the rock for the tide's rising. The path the stone must take resembles a "W", and when you finally position it in the sun's light, it will take in the Sun's light and fall into the water. Now you can bind the Sunstone anywhere in the Temple, since it no longer floats upon wood, and thus explore the dark waters of the Temple.
In the dark zone you have to open a door by singing the note that it displays. Use a Shield song or kill the crab done there before opening the Song Gate, or else take heavy projectile damage from it. After opening the Gate, the water will become less dark by an exit. It leads to a cove in the Veil, in which an Ancient Turtle lives. A Save Crystal and a large clam lay together to the Turtle's left, try interacting with the clam in different Song forms after saving. Below the Turtle is a Fish Form hole which eventually leads to the Octopus Miniboss- come back for it later, it's uber tricky to fight it before the Suhn Temple's God/Boss.
Re-enter the temple and pick up the Sunstone again. A second tide-changing gear must be activated after maneuvering through Gear Fish and between large, slow, and invincible Gear Bugs in order to reach the final Save Crystal before the Lumerian God, as well as... A parkour obstacle course to a secret exit. That exit leads to a Lost Memory, but don't get caught up with it for too long. To reach the Boss, there's a horrible obstacle course with a finicky tide and large, red, thorny balls. There is loot in the ceiling (minimap) but don't try snagging it until you beat the boss. If you get caught mid-tide change and start bouncing between the balls, you will lose health faster than you can sing 5 songs. It REALLY HELPS to acquire the Natural Form, because it is immune to the damage these cause.
The obstacle course, which is essentially a hallway, ends in a large chamber with the plants near the roof and settled at the bottom. This is the Lumerian God's fighting arena, and he is a tough dude to slay. He is wormlike, chasing you through and jumping out of the water to bite you. He does high damage, and it's worse that he can suck in and spit out the water, changing the tide for the battle, bringing you near to him or pushin you into the red plants. He has high health, and when 1/3-2/3 of it is depleted, he gets faster, starts belching projectiles at you, and some fast-paced music begins playing. Tough cakes, Regeneration, and increased Energy Shot power are all great for taking him down, but see if you can use the Natural form to trap him and/or stop inconming projectiles. Beast Form risks lots more health than need be. He's beefy and brawny- try to get a pet's help... Or someone else's to help subdue the boss.
After killing the God and witnessing its children's demise, you learn the Sun Song to activate the Sun Form, revealing what's in most dark places with a "flashlight" beam and a burst of light that illuminates the are around you for a small time? The tide for that area will rise above where it originally rose. Now you can more safely loot those missed Song Plants, and when all done, return to the arena and look for a hole in the ceiling. You will jump out of that area into the one with the Ancient Turtle and Fish Form hole, and if you've seen 2 Turtles you can skedadle without retracing your steps.
There is a wall that you can swim though to get a treasure, in the room just below the Sunstone's starting point. Bring the crystal (or another source of light) there once it's infused; there's also a glass-covered Song Plant just inside. Also, check the Gear Bugs' pipes and be prepared to unclog the Gear Crab to reach a SECOND treasure.