In order to access the lower part of Hollow, you normally need Bow in order to drain the water on the way to dash tree. This water stay dirty, even if you found Clean Water. However, the lower part of Hollow isn't flooded so you can swim through the dirty water and get all its pickups without having to drain the water.
The amount of health you will need depends on the difficulty you are playing on. This amount can be greatly reduced by having either Water Dash or the Resilience shard. You can also use godmode to remove the health requirement entirely.
Before the first lever
Here is how the scene looks like before pulling the lever which let you access the room with the rock puzzle.
The black part is the dirty water itself, you can clearly see that the bottom part of the scene isn't flooded. The goal is to swim through the dirty water in order to access the lower part. Midway through the dirty water is a blue rectangle, which is a checkpoint you can activate even though it is in dirty water.
Depending on which difficulty you are playing on, you will need more or less health in order to do so, needing as little as 3 healths in the easy difficulty. If you have a lot of health you should be able to swim through the water without any difficulty, as long as you know where the walls which you need to avoid are. When you have less health, you want to activate that checkpoint because, when the game respawns Ori, it gives you some of your health back. This extra heal makes it possible to finish down swim even with very low amount of health.
After the first lever
If you pulled the lever which let you access the room with the rock puzzle, the dirty water is pushed back, making the swim slightly different.
The movement you need to do in this state is easier since it's mostly straight down. It's also easier to abuse the checkpoint since you can't miss it and it will also respawn you outside of the dirty water. There is however two main problems with this state: the breakable wall which leads to an energy cell as well as the path leading to the Spirit Trial and its health cell are both underwater which make them harder to get. You can still break the wall with Water Dash and the right part of the Spirit Trial's path, just after the checkpoint isn't under water.