Guardian Eye
The Guardian Eye gives you the ability to shoot a laser at the entity you are looking at, within a 15 block radius. Note however, that in order to do this you need to hold down a keybind.
The item can be obtained as a rare drop from Guardians.
Dolphin Fin
The Dolphin Fin affects you with the Dolphins Grace effect while swimming.
This allows you to explore the water more efficiently.
This item can be obtained as a rare drop from Dolphins.
Drowned Lung
The Drowned Lungs is one of the most powerful augments.
They allow you to breathe both underwater and on the surface
This item can be obtained as a rare drop from Drowned.
Eldritch Heart
The Eldritch Heart will increase your Regeneration under water.
While this might not sound too overwhelming on its own, it is worth noting, that the eldritch heart is also required for the Augmentation Station.
This item can be obtained from Elder Guardians.
Next: Augmentation Station