Vodkaholic, no harm in going along. If you are lucky enough to be offered one of the jobs, then you can make a much more informed decision then, because you will have a greater insight into the company.
Ground Star, you need to be able to swim 25m unaided. Most airlines will make ou do this with a life jacket aswell. Some airlines make you swim fully clothed, with sneakers/trainers. 25m seems to be the standard in the UK, in Australia, you need to swim at least 50m!
However the survival statistics for a heavy jet making open water ditching are fairly poor, so probably the only time you will ever need to demonstrate your ability is during trainging! What a pleasant thought to leave you with... sorry