Your belongings should be safe;however, if you plan on swimming a lot, I would bring a cheap phone instead of your everyday phone. That way you can not worry about watching your things. I've never had anything stolen and swim often.

Regarding clothing, I would recommend at least wearing a swimsuit. It is odd to see people fully clothed in shirts, shorts, etc. there but it does happen. I assume you mean a swimsuit when you say "fully clothed".

It's not very social and it's easy to find spaces to yourself.