In the story I currently write, I have a character who has been a passionate swimmer for decades. Now, she goes to space. I wonder how she would adapt to weightlessness, and would her swimming experience help?
I don't see a specific reason why swimming would help someone adapt to weightlessness, although space health and "how well someone can adapt to a microgravity environment" is still a very active field of research.
That said, generally someone who pursues athleticism with dedication is going to be healthier and have a generally better quality of life. They're likely to have a stronger immune system, and be more resilient. Additionally, the discipline required to pursue athleticism with regularity might translate well to the fitness regimen required for long-term exposure to microgravity environments.
If you are looking for sports that might offer an edge:
Scuba diving or other underwater activities: Someone who's spent a lot of time underwater might be more skilled at "thinking in 3d" and have better spatial reasoning or navigation instincts/skills in a microgravity environment.
Gymnastics, Acrobatics, Martial arts, Dancing: These all have a heavy focus on body and kinesthetic awareness along with movement and flexibility. Knowing where all your limbs are and having the dexterity and flexibility to, for example, grip a handhold with your foot might be useful on a space station. Furthermore, these sports often teach specific movement techniques that can be used while flying or falling that might be applicable to a microgravity environment.
Rock climbers: Upper body strength to pull yourself around or tightly grip on even the smallest of ledges might be a useful skill aboard a space station, although generally astronauts have problems maintaining strength because everything is so easy.
Skydivers, bungee jumpers: Probably the only group of people besides astronauts that have really any significant experience in free-fall/microgravity
Swimming is excellent exercise in any case, but it does not really simulate weightlessness. If you simply do a dead-man's float you will be weightless at equilibrium, but in actual swimming more of your body is out of the water than would be the case in this equilibrium. So you still have net weight, which is supported by the currents you create around you in the water. Having to create this support under your own power actually makes swimming more like mountain climbing than space travel.