Three classes of Underwater Protection upgrades exist, B, A, and S. It appears that C-class is non-existent.
The B-class can be found at Space Stations, while A-, and S-class, are both a rare reward from Damaged Machinery.
For those curious which class they actually have, they can be distinguished as follows, when already installed:
- Name adjective: Backup, Secondary, Upgraded, Efficient, or Self-Cleaning
- sub-title: Underwater Oxygen Upgrade
- description: A significant upgrade for the Aeration Membrane
- Name adjective: Supercharged, Extreme, Saturated, Organic, or Pressurised
- sub-title: Significant Underwater Oxygen Upgrade
- description: An extremely powerful upgrade for the Aeration Membrane
- Name adjective: Hyperpressure, Deep Water, Abyssal**, Microbubble, or Pure Oxygen
- sub-title: Powerful Underwater Oxygen Upgrade
- description: A supreme upgrade for the Aeration Membrane
For each of the above, the adjective is followed by one of the following to make up the full name of the upgrade:
Air Filter, Air Harvester, Breath Recycler, Gill Unit, Gas Pressuriser, Air Circulator, Lung Replacement, Gas Pumps, or Rebreather
For example, I used to have an A-Class ‘Extreme Gas Pressuriser’ until I found an S-Class ‘Abyssal Gill Unit’. Interestingly enough, my A-Class never showed the adjacency outline, while my S-Class does.
Only one upgrade will function, similar to the various other protection upgrades (Thermal, Toxic, Radiation). There appears to be no point in installing multiple, even though they indicate to allow up to 3 max, with an overload warning if 4 or more are installed. I am unsure if this is intended behaviour or a bug, although I assume the former. Protection Upgrades also appear to trigger a bonus when adjacent, although I am unsure if or how this is translated in-game.