At the beginning of the level, use the animal statues powerup to sink through the water faster. It's nicest to get Pitch here because of its horizontal speed. Conversely, Kine sucks because you can't move, and I'm still not sure what the best way of dealing with that is. The remaining four friends are more or less equal, falling in between Pitch and Kine.
Throw away the powerup in the next room and inhale one of the Floppers. By holding the Flopper above your head, you can go through most of this room really quickly, but there are many obstacles in the way–go too high and one of the boulders will hit your fish, and go too low you run into a lot of enemies. The standard strat is to float up to the height shown in the top-left, allowing you to avoid everything. I like to go lower, as seen in the bottom-left, because I'm scared of the boulders. While you get to hold onto the Flopper longer at this height, you'll also run into one of the enemies (bottom-right). It is unclear which is faster.
In this room, the objective is to swim up without touching anything and grab Shard 1 at the top of the room. There are currents pushing you horizontally, with the top-most one flowing to the right. As a result, we need to stay towards the left side of the room. The clip on the left shows my preferred route, though it's possible to essentially hug the left side the whole time. In the beginning of the next room, there's a slope that you can slide off, saving a little bit of time.
Later on in the same room, you approach a pole with Shard 2 off to the bottom-right. You can slide down it quickly by simply double-tapping and holding down. Turning around on the pole can be done without losing any vertical speed. The quickest way is shown in the top-left, where you can inhale before getting to the bottom. Note that pressing A here doesn't work as well because it'll cause you to jump a little bit. The safer way involves just sliding to the bottom and running along the ground. Immediately after, inhale the Blowfish and proceed to the end of the room, swallowing only at the end. The enemy is swallowed here because going through the loading zone cancels a bit of the swallowing animation.
An easier Blowfish is one of the ones right before the pole (bottom-right), but swallowing it is really slow. Furthermore, pressing B on the pole needs to be done lower because the recovery animation with bomb (which causes you to rise a bit) takes longer.
The Blowfish miniboss has a quick-kill that involves getting stuck in the alcove (left). The clip shown here shows the riskiest way of doing it, where you start moving towards the exit after the second bomb. To stay up in the alcove, alternate between pressing A to swim up and B to throw a bomb. Make sure to only tap B, otherwise you'll hold the bomb up and it'll hit the ceiling. In the final room, stay close to the bottom, avoiding the random boulders and Gordos. In between the second set of Gordos is Shard 3 sitting in a pit.
If you lost bomb before the end of the level, you can quickly re-enter the stage to grab a Pedo (yes, that's its actual name) before heading to Acro.