Jump to Beedle's ship, pull out the Wind Waker, and perform a Wind Waker Dive. Wait a second, and then exit out of the conducting menu. Swim over under the entrance to the Deku Tree's chamber and wait to surface. When you do, you should surface above ground and in the loading zone.
A roll clip can be performed at the bottom of Forest Haven in order to clip under the island and swim to the loading zone. However, this trick is somewhat precise and also requires Beedle's ship to move slightly, making it only a small timesaver in a Wind Waker HD speedrun.
Since Wind Waker diving only works in very few places in Wind Waker HD, one alternative to a Wind Waker dive in a Hyoi Pear dive. However, since this requires the purchase of a Bait Bag, this technique is not faster in a speedrun.
It is possible to enter the waterfall where Makar is located before the Wind Temple by using only a jumpslash. This is particularly useful if you want to grab forest water first, as you can then follow the water down and use a late jumpslash to bypass the waterfall. Though it is possible to also jump from the platform near the post and get behind the waterfall with a jumpslash from there, it would be faster to simply grapple the post from bellow.