When I used to fly in a Martin Baker product it was pool drill (dropped off a crane) every 8 months, and sea-drills (thrown off a lifeboat, free helicopter ride an hour or so later) every 2 years. No pass, no fly.
I agree that you don't *need* to be a swimmer, but you do need to be able to keep your head and act as trained when immersed in water. Learning to swim is a pretty reasonable way of helping to achieve that.
Incidentally, for some years, I used to be a canoeing instructor - regularly teaching capsize and rescue drills. The minima set by the British Canoe Union was the ability to tread water for 2 minutes (in your kit, including buoyancy aid) and swim 50 metres - hardly a high standard of swimming ability. But, people who were not confident swimmers often (not always to be fair) panicked when capsized, becoming a liability to themselves and to a lesser extent other people.
G