As a material
Water is commonly found in bottles and fountains. Drinking water relieves exhaustion. It can also be used to wash other liquids off of equipment or to put out flames on your items and body, if you can figure out how to spill it properly.
As an environment
Water tiles cannot normally be traversed by the player, as you cannot swim. However, water can still be crossed by any creature that has the ability to swim or fly. As such, you can cross water if you can acquire Levitation or Swimming status effect.
You could also try to polymorph into a swimming/flying monster, but be warned that should you return to your usual flightless, unable-to-swim state while over a water tile, you will die instantly.