Floating: It's like if flight and swimming had a lazy baby. It is an uncommon ability, usually seen from the Spectral armies of Therodos or other ghosts, making them slightly more mobile than other undead options.

Floating works underwater.

Specifically, floating allows:

• Immunity to Earth Attack, Earth Grip, and other spells that specifically don't say "does not effect flying and floating units".
• Does floating grant immunity to tangle vines?

Despite the description, Floating does not in fact allow crossing rivers, as shown by the following image. Notice how the province across from the river is not highlighted like the non river crossing is: