It's only 20-30 miles across, and the water was quite warm during the time that they were trapped, wasn't it? When they were considering the possibility that they'd only get 45 thousand men back, did they ever consider just having the men take a go at swimming across? Assuming an average swimming speed of 2 miles an hour, couldn't they have swam it in under a day, accounting for rest periods? Would the Axis even bother trying to bomb hundreds of thousands of men all spread out trying to swim across at once?
EDIT: Okay, now that I've looked it up, it looks the water can get as cold as 59 degrees F, and only as high as 65 F. 65 degrees is still pretty chilly by pool standards. Would that be cold enough to cause hypothermia if you're submerged for a day?
There are several reasons why this was not done. Firstly, there were not enough life vests available. At the start of the war, the Royal Navy did not issue any individual life-saving gear, relying on boats and liferafts. It was only after significant urging by the commander of Home Fleet that inflatable lifebelts began to be issued. The lifebelt was intended for short-term survival, and was not suitable for long-term wear in the water. It did little to support the wearer's head, so if you were exhausted or unconscious, your head would fall forwards and you would drown. The RAF did issue a proper lifejacket, but these were only issued to aircrew. Neither of these pieces of lifesaving gear were available in sufficient numbers to save the BEF at Dunkirk.
Secondly, swimming was not a common skill amongst British soldiers. Swimming was a relatively rare hobby in the cities from where the Army drew much of its men. Despite the establishment of many swimming baths in inner-city areas in the late 1800s and early 1900s, swimming was not well-established. For the RN, where swimming was seen as a useful skill, basic training included swimming lessons; not so in the Army. Even for experienced swimmers, the Channel crossing is a difficult one. The strong tides and cold water make it tricky, even when crossing the Channel at its narrowest point. Dunkirk is a considerable distance away from the narrowest point - rather than being 21 miles from the UK, it is ~42.