「海」 (umi) means "sea" or "ocean." You use it to talk about the large body of saltwater that covers most of the Earth's surface, or a specific part of it, like a beach. Many people enjoy going to the 海 for swimming or relaxing, especially in summer.

地球(ちきゅう)の大部分(だいぶぶん)を覆(おお)う広(ひろ)い塩水(えんすい)のことです。泳(およ)ぎに行(い)く場所(ばしょ)です。