but he cannot swim indefinitely. 但是它不能永远游在水里

230. He is forced to head out into deeper water.
     它必须进入更深的水域
231. His giant front paws help him to fight the ocean currents.
     它那巨大的前爪有力地拨动着海水
232. He seems at home in the sea,
     在海里它似乎是那样地悠游自在
233. but he cannot swim indefinitely.
     但是它不能永远游在水里
234. He will drown if he doesn't find land somewhere in this vast ocean.
     如果在这片浩瀚的海洋里找不到陆地 它就会淹死
235. Walruses are now gathering on low-lying islands.
     海象正往低洼的岛屿聚集
236. They gave birth on the sea ice,
     它们在海冰上产仔

来自《行星地球 第一季 第六集 Planet Earth S01E06 冰雪世界》第 233 行