See the Cabana Deed Skip page.
Simply use a skull hammer jump attack to hit the wooden pegs through their respective adjacent walls. This avoids having to navigate your way through the sewers in order to get past the gates.
Use a sidle to clip part-way through the bars and then finish clipping by rolling. This is particularly useful if you are skipping the skull hammer in a Low%, and is the only part of the Cabana Sewers that otherwise requires the skull hammer. In any other case, it should not be used as the above method is faster.
You can alternatively skip the Skull Hammer Requirement with a very difficult L-Slide Clip to go OoB, and from there leaf to the last ladder. This also skips all Cabana Sewers at the same time.
It is possible to clip out of bounds through a seam right as you enter the sewer with a bomb push clip. Some solutions are known but there are currently no RTA setups. For example, you can stand at X=574.59515380859 and Z=-565.53485107422 with angle 28672-28687, and drop the bomb on animation frame 1 to clip. Once OoB, you can leaf over towards the end and pop up top using the water.
After obtaining the treasure chart in the Cabana, backtrack to the hole that is located just behind the chest (follow the wall on the left of the chest to find it). Drop down the hole and sidle behind the ladder. Pull out the hookshot once you're behind the ladder. You wil notice that you are now able to aim out of bounds, so just aim at the only hookshottable ladder you can see and get pulled to it. Now repeat the process again with that ladder to hookshot to the chest containing 100 rupees. If you want even more rupees, store that chest. Thanks to chest storage, you can now hookshot through the wall on your left to another ladder. Now, jumpslash and you will notice that you will start swimming. You can now clip out of bounds and leaf to reach the pots containing around 70 rupees. Then go towards the crawlspace to once again hit water and clip back to the entrance. Leave the area to collect the rupees from the chest you stored.
You can obtain the Treasue Chart contained on the chest on top of the waterfall without the leaf and the hookshot using a wind waker dive to clip under the island, then swimming under the island and under the small pool to clip back up to where the chest is.
Fire rings can actually be extinguished with water type damage in the Wind Waker. This however cannot normally occur, because the target collider of the fire ring is not active during the bottled water pouring cutscene, and the attack collider of the water only exists during said cutscene and disappears after. Using Forest Water, and by pouring water on the exact frame forest water expires (see Forest Water Storage), it is possible however to make both colliders exist at the same time. Doing this normally softlocks Link as it makes him unable to move, but this can be combined with a bomb boost to escape this softlock state.
Thus, to enter the cabana sewers without the grappling hook, first perform the Item Swap glitch in order to pull a bomb inside the cabana (which are normally restricted). Then immediately pour water on the fire ring on the exact frame forest water expires. This will extinguish the fire ring, and the bomb will boost Link into the loading zone. The timing is really tight.