Before the encounter begins, interact with the Oxygen-Rich Membrane either at the start of the trash towards the boss or nearby the boss platform, this will give you additional swim speed and allow you to breathe underwater.
When you are underwater you will have the Darkest Depths debuff which will reduce the healing you receive by 100%. To be healed again you will either need to get the Bioluminescence buff or Radiant Biomass.
Touching any Jellyfish whilst swimming will apply a 8 second DoT.
[Heroic] Touching a Jellyfish will apply Bioluminesence to you. Avoid the Jellyfish whilst swimming across.
When you arrive here the boss will still be channeling Cavitation. If this cast finishes it will deal heavy raid damage every 2 seconds. However you are able to deal full damage to the boss during this period, and pre-collect Bioluminesence from a pufferfish.
The rest of the mechanics are the same as Platform 1, however there is an open gap between the two ranged platforms and the melee platform.
When the boss reaches 100 energy it will move to the final platform.
The boss has no energy on this platform and the raid only has the two pufferfish to use. This is the soft enrage of the encounter.
There are more gaps in the platform and the overall space for the raid is smaller.
Piercing Barb - The boss will target a ranged player and cast a beam on them. The damage from the beam is split with all players inside of it, however the clump detection is still active so you want to spread out along the beam.
Tank Damage Profile
Swap when your Radiant Biomass debuff expires to be healed
Stay in range of the boss when its active
If you are using the punchcard trinket, the Slipstream Generator (Blue Card) works on this encounter increasing your swim speed significantly.
Pre-place your Transcendence near the boss so that you can get back into melee range after the push back
Vengeance Demon Hunter
Infernal Strike to get back in range of the boss after the push back
Blessing of Spellwarding can be placed on other players to avoid them taking damage on the final platform if you run out of Pufferfish
Rallying Cry can be useful when the raid has run out of Pufferfish
Acquatic Form can be utilised on this encounter to allow you to move between platforms faster.
Wild Charge will also make you swim even faster.
Blood Death Knight
Death's Advance can be used to negate the push back entirely