Summoned from depths unknown, the Blackwater Behemoth lurks in an abyss so dark that not even the naga dare to venture into it. Only Azshara is capable of taming the beast, and she has trained it to faithfully guard her treasures.
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.
On each boss platform there are two Shimmerskin Pufferfish, and when killed it will spawn a large 
Bioluminescent Cloud. Moving into this cloud will give you the Bioluminesence buff.
The tanks will not need to go into the cloud as each boss swing will apply Radiant Biomass applying the same effect for 15 seconds.
The room is split into three platforms, and when the boss reaches 100 energy it will move to the next platform.
Platform 1
Bioelectric Discharge - If no players are at the boss once it has been engaged it will pulse damage to the entire raid.
Feeding Frenzy - For every successful hit the boss lands against the same target, it will gain 8% attack speed. This resets when it successfully attacks a new target. You will passively manage this by ensuring that you keep Radiant Biomass active.
Toxic Spine - This will be cast on a timer to several players in the raid and deal ticking nature damage to them.
Shock Pulse - The boss will cast this twice on this platform. It will push back all players during its channel and at the end of the channel deal damage to all players based on their distance away from the boss.
Bioelectric Feelers - The boss will scan the raid every 4 seconds to check if any players are within 5 yards of each other. If they are, everyone in this clump will be hit by this ability.
You are able to spread out vertically thus giving you significantly more encounter space.
When the boss reaches 100 energy it will move to the next platform and begin to cast 
Cavitation.
Swimming
At the edge of every platform there is a 
Slipstream which will give you a swim speed increase.
On normal mode it will remove your Radiant Biomass/Bioluminesence buff.
[Heroic] Your Radiant Biomass/Bioluminesence will not be removed by the Slipstream, so you will need to wait for it to expire.
If you have Radiant Biomass/Bioluminesence whilst swimming over open water you will receive the 
Gaze from Below debuff, this lasts 5 seconds and if you are over open water when it expires you will be eaten and instantly die.
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.
Platform 2
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.
Platform 3
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.
Mythic
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.
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
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
[Mythic] Blessing of Spellwarding can be placed on a Piercing Barb target that does not have an immunity to allow them to solo soak.
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.
Death's Advance can be used to negate the push back entirely