Early Master Sword (EMS) is a sequence break that allows you to get the Master Sword before completing the Tower of the Gods and the first visit to Hyrule. This is done by activating the cutscene in Ganon's lookout during the second visit to Forsaken Fortress which will give you the Master Sword if you do not have it already. Although this may seem incredibly useful, it leads to a dead end as discussed below. However, since the Master Sword does twice the damage of the Hero's Sword, it can be useful in speeding up battles in some individual level runs.
Note: It is essential to have early bombs to get the Master Sword early and it is also very helpful and recommended that you also have the deku leaf and Tingle Tuner!
Although it is possible to reach Forsaken Fortress a second time via zombie hover from Windfall, this is an extremely long hover that will take multiple hours to complete and is only possible if you have the Tingle Tuner. However, with the discovery of dry storage it is now possible (and much quicker) to use superswim to get to FF2 early.
Superswim is a much faster way to access FF2, and makes this sequence break more practical for use in Individual Level speed runs. Get dry storage off of the rocks to the right of the docks on Windfall. When building up speed for your superswim, you may want to start off in the water by the docks, as you will almost assuredly get more than one air refill. However it is also very likely that you may hit land or grab the dock and have to restart your superswim.
Once you have built up a fairly substantial amount of speed and your air meter begins to turn red, use your sea chart to make sure that you are aligned so that your back is towards Forsaken Fortress and release the control stick. If you've built up enough speed, as you enter the quadrant of Forsaken Fortress, you will fall and get an air refill. If not, you will have to try to swim to one of the indentations in the rocks along the fortress's base where you can refill your air meter.
Now comes the tricky part: pulling yourself up onto the rocks so you can fix the crazy camera and get into FF2. There is a rock the is slanted into the water that you can use to pull yourself up (refer to the video below). Hold L so that you can swim here without charging up a superswim, and then hold straight up on the control stick to spin around and grab the ledge. Once you've pulled yourself up, pull out the wind waker to fix the camera and use a wind waker dive to get into the fortress and swim over to the inner-fortress loading zone.
Note: If you save and quit/reset before reaching the Helmaroc battle, you will be taken to the King of Red Lions who will then automatically sail you back to Windfall. Avoid making this mistake!
A very interesting (as well as important) result of visiting FF2 without the Master Sword is that it is impossible to damage Phantom Ganon. It is therefore impossible to obtain the Skull Hammer and complete the dungeon as usual.
Instead, you will have to use a wind waker dive to get past the Phantom Ganon battle and use zombie hovers to get to the Helmaroc battle (which must also be skipped as you don't have the Skull Hammer). If you mess up the wind waker dive getting into the fortess and activate the entry cutscene, there is another spot at the top right of the first set of stairs where you can use the rocks to wind waker dive and get to the loading zone.
Once you've gotten inside and bypassed Phantom Ganon, head to the right a few rooms until you get outside by the search light. From here, you can either attempt a hover directly to the loading zone of the Helmaroc battle (using the telescope to align with the big doors will help), or hover up near the doors, heal with Tingle, and then do another hover and use a red Ting and the deku leaf to reach the loading zone. Multiple methods to skip Helmaroc King can be found here.
The only confirmed way of doing Early Master Sword in TWWHD is by spawning on layer 8 in the overworld. This can be done by skipping the gossip stone on your first visit to Forsaken Fortress and then then coming back a second time, using the pirate ship, to reclaim the gossip stone and beat Forsaken Fortress a second time. Doing this causes Link to spawn on layer 8 in the overworld which is a result of the game expecting the cutscene where Link and the King of Red Lions talk for the first time. This cutscene does not play but the game still sets the layer for the cutscene anyway.
Although you will not encounter and problems with getting the Master Sword early at first, there is a major dead end that occurs which prevents you from being able to destroy the barrier leading to Ganon's Tower. Once EMS has been used, it becomes impossible to draw the Master Sword from what is normally the first visit to Hyrule. Though this may not seem important, drawing the Master Sword from Hyrule 1 is one of the flags that triggers the breaking of the barrier cutscene in Hyrule 3.