どん底[donzoko] very bottom
井底[seitei] well bottom
海底[kaitei] bottom of the ocean
海底ケーブル[kaiteikeeburu] undersea cable
丸底[maruzoko] round-bottom

基底[kitei] base
靴底[kutsuzoko] shoe sole
黒底翳[kurosokohi] black cataract
根底[kontei] root
周知徹底[shuuchitettei] (something) known to all

海鷂魚[ei] ray (fish)
井の中の蛙大海を知らず[inonakanokawazutaikaiwoshirazu] parochial
一天四海[ittenshikai] the world
雲海[unkai] sea of clouds
沿海[enkai] coast

遠海[enkai] deep sea
黄海[koukai] Yellow Sea
温海[atsumi] warm sea
海の幸[uminosachi] seafood