at sea，on the sea，seaborne ；

[例句] 把她的人生比作一次海上航行不足以说明她丰富的经历。 The comparison of her life to a sea voyage simplifies her experience.

[例句] 一艘捕鱼船和一艘货船在波涛汹涌的海上相撞了。 A fishing vessel and a cargo ship collided in rough seas.