Water
From Teletraan I: The Transformers Wiki
Water is a chemical so rare even its existence is suspect on Cybertron, or at least so claims Ratbat. It cures Scraplets. The Cure!
Cybertron's surface is covered by at least a few seas, but these seas are not of water. The Rust Sea, for example, is a sea of, uh, rust. There are chambers deep inside Cybertron which also run with liquid, but they have been confirmed to be not water.
Some Transformers, namely the Aquaspeeders and Stormtroopers, use high-pressure water-cannons as armaments. This is after they apparently re-discovered water, or something.
Some Autobot guy states "The two things I hate, fire and water." More Than Meets the Eye, Part 1
Hound is shown to have a mask for breathing underwater while Megatron understands how a hydroelectric station works. More Than Meets the Eye, Part 2

