Are they comfortable to wear for a long time?

Yes! Since they don't fit into the ears, they avoid the pressure and discomfort of in-ear headphones.

Can other people hear what I'm listening to?

A little, yes. At high volume, the sound may be slightly audible to people nearby.

Can you swim with bone conduction headphones?

Yes, but it depends on the model. Some models are waterproof (IP68) and work with built-in MP3 storage.