There are several ways to enter Lakebed Temple early.
If you've obtained the Master Sword early, it's possible to skip past the windy underwater portion of the first room by save warping.
From the end of the bridge, pull out the Gale Boomerang and target the collumn towards the left side of the door and do a long jump attack. From here, climb onto the fallen boulder, and target near the torches on either side of the door and use another long jump attack to reach the next plaform.
Get on the end of the bridge and use the Gale Boomerang to lure a Helmasaur towards the edge of the above platform. Once he is near enough to the edge for you to target him, do so, and pull out you sword. Back up a bit and do a long jump attack off of the Helmasaur. If he is too close to the edge, you will have to wait for him to turn around otherwise your jumpslash will bounce off of his armor and the recoil will prevent you from landing on the platform. From here, turn into a wolf and climb to the crest of the fallen boulder. Align yourself, and jump to grab the ledge of the next platform.
By using the Boomerang it is possible to throw a bomb against the pillar on the ceiling making it drop down. This leads the way to the Water bomb chest up there.
Right before the bomb hits the pillar, jump attack to get on the platform and save time.
You can also get up the ledge without knocking down the stalactite first.
Stand on the edge right next to the door leading to the main room and aim at the corner of the platform in front of you. Jump as soon as the boomerang goes out of bounds.
This is not faster than knocking down the stalactite from below and should only be used if you run out of bombs.
One time consuming task that occurs in the Lakebed Temple is emptying your bomb bag of normal bombs so to obtain water bombs from chests. Instead of throw the bombs into a body of water, head over to one of the waterfalls in the stalacmite room. By standing under the water while trying to pull out bombs, the bomb's fuse is put out immediately allowing for incredibly quick disposal.
From the entrance to the swiveling staircase room, head over to the top of the staircase and pull out the Gale Boomerang. Target once just right of the chandelier on the ceiling and twice above the overhang on the right. Long Jump onto the staircase's banister, and then to the ledge. This is a lot faster than have to move the staircase multiple times.
It is possible, using long jumps, to get the clawshot much earlier than was intended without ever having to go to the second floor of the dungeon and change the water level. Go to the hanging platforms room and run to the ledge of the top platform. Pull out the Gale Boomerang and target above the doorway on the left. Use a double long jump to jump to the hanging platform, and then to the door.
Please note there is a faster way to enter Deku Toad then this LJA which uses Early Deku Toad
After doing the above trick, you can get over the railing in order to get to the deku toad faster. Jump attack on top of the chest and pull out the Gale Boomerang. Target slightly to the right of the pillar. Now simply use a long jump to get onto the railing and blow up the rock. If you have gotten the Ball & Chain early, use one more jumpslash to get past the previously mentioned pillar, and you can use the Ball & Chain to break the rock if you don't have water bombs!
It is possible to get the Heart Piece outside the Deku Toad before getting the clawshot and still being able to get out which saves 14s in 100% over getting it after. First stand where shown in the video, and then line up the cross-hair just to the right of the pot (pot cannot be broken), then go up and target above the platform, then target the floor nearby the pillar with the gate and heart piece. Now LJA to to pillar with the pot once the boomerang reachs the first target, then LJA again over to the nearby floor when the boomerang reaches the second target.
Now target the pot with the boomerang to grab it and place it on the pressure plate, go into the gate and kill the lizalfos with a Jump-Attack into Spin Attack. Open the HP and then hold L and roll forward once, then shield bash once, throw the boomerang to lift the pot off the pressure plate and start closing the gate, then shield bash while the gate is closing to get clipped OoB. You can use the Iron Boots to void out faster.
If you get trapped in the area with the heart piece it is possible to escape without death or savewarping. First sidehop up onto the back of the open chest, then look towards the torch on the wall and Jump Attack, if the angle was correct you will clip up onto the torch. From here you can do two targets to perform an OoB LJA to escape over the gate.
By doing 3 LJAs (GCN Only) in the western gear room, you can reach the chest containing a heart piece without having to change the water level/activate the gear.
Here is a back-up method if you fail you can climb up and use this set-up
Drop down into the center of the key room, and head toward a patch of vines. Once you're standing next to them place a bomb behind you and Vine Clip through the wall. This can be done even if the water level has been raised by using Water Bombs and Iron Boots. Once you clip out of bounds, you must un-equip the Iron Boots by equipping an item over them.
Once out of bounds, equip the Iron Boots to sink below the floor, then swim toward the GCN left side of the pillar in center of the room. At the center, the game physics will start to behave erratically. If you pass underneath some open floor, the game will try to warp you back in-bounds. This is useful for getting high enough to reach the boss room trigger, and can even put Link back in-bounds inside the small room behind the boss door. Once you're high enough, head to the center and drop down. If you don't have the Zora's Armor, it's highly advised that you equip the Iron Boots before dropping into the Boss's Room as you will drown just before you reach Morpheel if you don't equip the boots before you surface.
It is possible to clip into the bridge by standing on the edge of the barrier and falling down onto the seam, from there with a perfect angle you can roll or walk up the bridge to the mid-point. You can then slightly face into the wall and clip, if you were at the right spot you will clip inside the boss room.
There are several methods of doing this but only two are really used lately. SVA found the the best known method of this clip which is a bit harder to learn but consistent. Clawshot up to the chandelier standing right next to the switch. Drop down, make a step to to right and sidehop.
After landing on the pillar, walk up and position yourself as shown. L-Slash slowly (don't mash!) until you drop down and backhop.
Then charge a spin attack and carefully go left until the outer end of Links beltbag touches the wall. Release the quickspin, backhop three times, let go of L and hold right to clip into the Boss Room.
Ekke325 found another way to skip the boss key that barely involves the chandelier. It is the fastest known method so far.
Enter the main room through the right door, jump attack once and then throw the boomerang with a target in the middle of the chandelier. Jump as soon as the boomerang reaches the target.
You will land on the stairway railing. Now make a small step to the right and sidehop twice. This will get you in the same position as with the setup above - continue as stated there.
After Collecting the heart piece put on the Iron Boots and sidehop left once, backhop twice, then unequip the Iron Boots. Now Sidehop (It will be small) then backflip down, now hold down until you fall into the bridge. L-Target Roll twice then look left into the wall slightly and L-Roll to clip
In 100% you LJA to Ooccoo after getting the 2nd heart piece. To do this don't move once you open the chest (or stand where you should be), then target above the overhang and LJA into the openning. If you target too far right you will hit the wall with your LJA and recoil all the way down into the water. It is possible to not target at all, just throw the boomerang at the spot and still instant LJA in time. NOTE: The video below target very far right, any further would have failed
An alternative method which is more risky and no faster is to stand on the edge of the chandelier and target at the two spots on the wall (not precise but could use vines as visual ques), once the boomerang goes out of bounds LJA (It isn't 100% consistent to go OoB hence why this method isn't recommend). It is possible to LJA so your jump attack breaks the pot, it is also recommended to do an auto spin in case you miss the pot