流水 ・ りゅうすい ・ ryuusui
Flowing water. A water pattern consisting of swirling lines. Large curved lines can indicate gently flowing water, while the addition of little swirls and/or wave like tentacles create a wilder flow. The pattern is almost always stylized as lines and can be used as an all-over design or as an element in combination with other designs.
Several patterns with different names exist that show flowing/swirling water. If the exact name is not clear you can call it ryuusui.
See also Kanzesui and Kourin-nami.