Water

Cybertron gave up water eons of your Earth years ago.

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.

(Note: Ratbat and other Transformers who are unfamiliar with water are likely confused; the several humans later managed to exist without environmental protection gear on Cybertron without drying up like old corn. Also, Ironhide mentioned his "water gun" right in the very first comic.)

From Teletraan I: The Transformers Wiki, a Wikia wiki.