Kenoco - I don't think you will find that there is any hard and fast answer that covers everyone. Each aviation regulatory authority in each country will have its own rules.
What Atropos says makes good sense though.
When I was flying on the UK register we had to complete dinghy drill but wearing a life jacket at all times, (Tech Crew), when we started long overwater flights - that was a while ago and the rules may have changed.
I have since flown for a Middle East operator and only Cabin Crew were required to do any swimming, (in the pool at the local Hilton!), and I have flown with a S.E Asian carrier who require all crew to complete dinghy drills and life saving but again, with a life jacket at all times.