Early Deku Leaf is a sequence break that allows you to obtain the Deku Leaf before completing Dragon Roost Cavern. This sequence break allows you to easily skip the bottle and makes Dragon Roost Cavern slightly faster.
After obtaining the Wind Waker, you can do a double storage superswim to Forest Haven in order to skip having to first beat Dragon Roost Cavern. In Forest Haven, you need to activate the Deku Tree cutscene in order for the Deku Leaf to appear. After you've watched this cutscene, damage yourself down to a quarter of a heart with either Chus or Bombs. Exit the inside of Forest Haven at the lowest level. When you are outside, hold your shield and do a backflip (this will kill the Octorok). Walk forward until you hit the little ledge in front of you. If you would like to make the hover more lenient, do a sidehop to the right at this point to change your angle. Place a bomb and then turn around and pull out the Tingle Tuner to call Tingle, along with your sword. After you call Tingle, quickly switch to the Red Ting, and after the bomb knocks you forward, begin a Zombie Hover. Once you've hovered near the highest platform, you can use your Red Ting for 20 rupees to heal yourself. The unique aspect of this hover is that you do not need to actually land on the platform that you're hovering to. If you're relatively close to it when you heal yourself, you'll fall into the water, void out, and spawn back on the platform. From here, you can enter Forest Haven. When you enter, do a sidehop and C-up, line up the B Button with the spot seen in the video below, and then hold forward. If done correctly, Link should have the perfect angle for Bulb Skip, and now you can obtain the Deku Leaf. Skipping the final bulb saves 5 seconds.
As an alternative to using the first-person camera (c-up) to get an angle for jumping to the bulb, you can sidehop right then do two dry rolls, and hold up-left during the second dry roll until you jump. If done right you should land in the bulb. The downside to this is that you need to get the angle for skipping the final bulb in the short time you have before you get spit out.
If you accidentally fail the jump to the bulb, there is a way to get back up without redoing the hover. Generally when you void out somewhere you will respawn at the entrance you used. It is possible to use a roll clip to jump out of bounds and respawn back at the top.
By hovering with a very specific angle inside Forest Haven, you can hover up to the deku leaf and grab it while dead to obtain it early. You can use bombs to start the hover, but it is also possible without bombs by using fall damage as seen below:
It is still possible to obtain the Deku Leaf without the Grappling Hook and the Tingle Tuner. To do it, you first need to get chest storage. A good place to get chest storage is on one of the rafts in either Boating Course or Five Star Isles.With chest storage, superswim back to Forest Haven. Thanks to the alterred collision, it is relatively easy to clip under the island. Then swim under the island until you get brought back up near the entrance loadzone. Make sure not to trigger the loading zone or else you will lose chest storage and a lot of time. Thanks to chest storage once again, you can climb around the entrance and perform repeated jumpslashes up Forest Haven. You will get enough height in order to hit the respawn trigger. Then simply wait until Link drops into the water to respawn at the top of FH, skipping the grappling hook requirement for the Deku Leaf.
The strat above can also be done swordless by acquiring a deku stick. The deku stick can be used to jumpslash your way up the island with chest storage. In order to obtain a deku stick, you must lure an octorok into shooting a deku baba, and time a tingle bomb to deal the final blow. Then you can throw the stick next to the Forest Haven entrance and proceed like before.
This is possible without the Tingle Tuner by manipulating two different octoroks into both shooting the boko baba in a very quick succession, but that is very hard/inconsistent.
This last method of getting the deku leaf early is considered to be a TAS strat. First, get chest storage and superswim over to Bird's Peak Rock. It is possible to swim up land with chest storage and ride up the island. While riding up the island, activate a Tingle Balloon and then shoot off. You will travel backwards very high above the ocean at very high speed. Enter the loading zone at Forest Haven to obtain the leaf early.