The sword should hit the pillar on the opposite side of the railing, causing you to recoil back into the Jabu-Jabu loading zone. However, if Link does not jumpslash quickly enough it can be a long swim back to setup again. Because of this the next method is still commonly used.
It is possible to jump too high, in which case you need to slide down a little bit and do another jumpslash to hit the loading zone. Getting to know exactly where to aim for requires a small amount of practice.
It should be noted that both of the above techniques can also be performed from the left side of Jabu-Jabu's head.
Entering Jabu Jabu as an Adult used to be incredibly difficult.
Newer and easier methods have been found in the years since its original discovery, however.
Using the setup shown in the video below and sideflip bombchu hovers, you can enter Ice Cavern as child.