The underwater area of Genshin Impact 4.0 doesn't only include normal enemies, but also notable mini-bosses. Considering there are two Crab and two Hunter’s Ray bosses, there are likely two Blubberbeast bosses as well, although, by the time of writing, only one of the bosses has been found.
Since each boss grants one achievement and Precious Chest, players can earn up to six Chests from hunting these mini-bosses. An extra achievement is also available from having a character beaten by the Hunter’s Ray boss, so there are seven Genshin Impact achievements in total.
Where To Find Underwater Boss In Genshin Impact Fontaine
Here's how players can find each underwater boss and how to beat them to claim the promised Chests in Genshin Impact.
How To Beat Ocean Circuit Judge And Iron Viscount In Genshin Impact
The Giant Purple Crab mini-boss in Fontaine’s underwater area is probably the most annoying one out of all the other mini-bosses. To beat the Giant Purple Crab in Genshin Impact effectively, players must hit the boss right after the enemy’s attack hit the players. This retaliation will deal about 3.5 times higher damage than a normal attack would, which will make the fight significantly faster.
The Genshin Impact boss has two types of attacks; the fish throw and body twirl. Both of these attacks can be retaliated, but the fish throw is far easier to counter as players simply need to aim at the boss and wait until the fish hit them before releasing the attack.
Once reaching a certain percentage of HP, the boss will summon a shield that can only be broken with this retaliation technique. First, deflect the hit from the fish, then swim away as the boss will send a bullet as their own way of retaliation. Before the bullet hits, immediately prepare the hold skill again and attack the boss when the bullet hits; repeat until the shield is broken to deal damage. Fortunately, if this part is too complicated, breaking the boss’ shield is not a necessity as the shield has a limited duration and the boss needs some time to regain it.
Ocean Circuit Judge Location
Ocean Circuit Judge in Genshin Impact can easily be found in an open area south of the Court of Fontaine.
Iron Viscount Location
Iron Viscount is a Genshin Impact boss that players will meet after solving the hot spring puzzle in Beryl Region.
How To Beat Fading Veteran In Genshin Impact
Beating the Purple Blubberbeast mini-boss in Genshin Impact requires players to identify the correct sonar attack to reveal the boss from its invisibility state. After Travelers first attack the creature, it will go invisible and several sonar pulses will appear all around. Within these pulses, one of them should have a slightly different movement, and if this attack is hit by the player’s Sonar Pulse, the boss will be revealed and immobilized for a brief second.
Fading Veteran Location
Fading Veteran boss in Genshin Impact can be found south of Elton’s Trench Teleport Waypoint, or directly east of Beryl Region’s Statue of the Seven. The creature is swimming in the water with darker color, between the two underwater canyons.
Dobharcu Lord of the Hidden Location In Genshin Impact
Head to Chemin de L'Espoir and go to the open space on the west to find the location of Dobharcu, Lord of the Hidden in Genshin Impact.
How To Beat Hunter's Ray Boss In Genshin Impact
To beat the Purple Ray mini-boss in Genshin Impact, players have to cut the connection when the two enemies establish one. Use the Ray’s cut skill to do this and deal large damage to both of the creatures. If the connection isn’t there, then simply attack it normally while avoiding the foe’s water slashes.
The Purple Ray underwater mini-boss always comes in pairs. They both can deal large damage if players are hit by their attacks, so it’s important to either dodge or bring characters with large HP like Nilou, 
Zhongli, Kokomi, 
Hu Tao, and so on. 
Lynette can also help as she can provide higher healing from nearby Recovery Orbs.
If a character falls for the first time to the Hunter's Ray boss, The White Path Between Two Rivers achievement will be rewarded.
Sword of the Gorge Location
Players can find the Sword of the Gorge mini-boss west of Thalatta’s underwater Teleport Waypoint, which exists northeast of Court of Fontaine.
The Fairy Knight Twins Location
Meanwhile, the Fairy Knight Twins location can be reached by using the Talent domain of Fontaine and heading east.
Genshin Impact is now available on Mobile, PC, PS4, and PS5. A Switch version is in development.