Not very, but once you start getting them you can become more prone. Swelling from infection can alter your ear canal to making it more likely to trap water.

Some might tell you earplugs as a preventative, but they can cause problems of their own, especially if you take them out and put them back in during a session. Or if they get dirty.

Swim caps will keep most of the water from your ears, and if you've longer hair I'd suggest a cap regardless.

Really you should be fine so long as you're not sleeping with water in your ear. Especially if you're in a pool and not open water. They weren't common when I was swimming.

Depends on the person. I've had friends who get them all the time from basically any water. Meanwhile I can flop around in stillwater or death smelling run off and I've never gotten one