Tri / Triathlon / Triathlons - do they float your boat?

Question - As I haven't been swimming for a loooong time now, how much on average unfit person can swim?

Tempted to do half in Poland while on holidays. Is 5 months enough to train to regain fitness (I can only run 10 km right now)?

I think 5 months should be fine. If you've got a good technique, you've nailed the hardest part of swimming. Just depends how fast you want to do the 2.2km. 3/4 Swims a week with a mix of drills and stamina sessions should be more than enough across 5 months. I find an early morning swim after a run the night before priceless in stretching out certain muscles (I am the world's slackest stretcher).