海洋生物学是研究水下生命的学科。

Marine biology is the study of underwater life.

现在，我能说些什么关于在水下行走的感觉呢？

And now, what can I say about the way it felt to walk underwater?

我需要在水下屏住呼吸。

I need to hld my breath underwater.

潜水员在水下使用氧气瓶。

Divers use oxygen tanks underwater.

潜艇可以在水下停留数月。

The subs can stay underwater for months.