I went below deck to just clear my head.

我去游艇下层 想冷静一下。

One minute they're on deck and the next they need to go below deck.

前一分钟他们还在甲板上，下一分钟他们就需要到甲板下面去了。

If a ship was in a bad weather and its sailors were sick, the crew would go below deck, to get out from under the weather to try to feel better.

如果一艘船遇到了坏天气、又恰巧船上的水手生病了，船员会到甲板下面，避免直接“在天气下面”（under the weather）。让自己感觉好点。