No one knows what the water will be like until you are in it. Also, for this extraordinary event, the Captain was able to turn the aircraft in the direction of the tide. Had the tide been on the flood? Then the water would have been very choppy indeed. Further, the lifejacket makes keeps you in the right position and more visible to rescuers - particularly if you are semi-concious or disorientated.
Put the jacket on and inflate when outside the cabin!