yes it is important.
In any complicated situation, you will be a f*ing nuisance and put yourself and others in danger when you can't swim.
Yes 99% of dives are happy sunny easy ones, but what if the weather changes suddenly, you come back to the surface in a current and wvaees, and the boat captain fails to find you?

there was a good thread here a while back about this and the fact that some guy had diving friends who claimed they didn't swim at all. Take what was said about that with a pinch of salt, but it did raise some good issues about the level of ability you need.

If you can, practise in the sea and not just in a pool. Sometimes you have to do the 200m swim test in the sea. They'll find as calm a spot as they can, but it's still not the same as a pool IMO.

so is that 10kg positive or negative Klaush? Floating in your bc is always easy. People normally get in trouble because they dont use their bcd properly, not because they are not a great swimmer. just dont take off your fins and bcd at the end of a dive, and leave your weights on, and expect to be able to swim anywhere. saying that, of course being a better swimmer is better imo.