Take a swim, feed those hungry steam turbines, dig a moat, or dig canals to connect bodies of water (useful for boat and ship mods).
Waterfill - replace dirt with water
Swimming - Players, vehicles, and biters can cross shallow water
Waterfill can only be placed adjacent to existing bodies of water - no spawning water from nowhere
Waterfill only produces shallow water, which biters can cross
Water will slow down biters (and players)
Minimum 3-tile width, since this can be used as a high-throughput water pipe for reactors