Not only does this vast body of water separate Kirigakure from all other countries and villages, but it's also where shinobi train their ability to walk on water. Typically, a shinobi tries to run from Kirigakure, across the ocean and back by running on water. A rescue team in a boat always stays closeby just in case the mission is a failure. The ocean is more for looks than for a nearly suicidal training session.