YSK if you're ever in a plane that crashes into water, DO NOT inflate your lifevest until you are outside the aircraft. Inflating your lifevest inside a sinking aircraft will cause you to float and become trapped.
from /r/YouShouldKnow
YSK如果你乘坐的飞机坠入水中，在你离开飞机之前不要给你的救生衣充气。在下沉的飞机上给你的救生衣充气会导致你漂浮并被困住。