I agree with Skander's explanation about "baigner". Obviously this is the transcription of someone's speech. The person who is speaking uses "ouais" which is commonly used in informal French. You don't say it if you're in a formal context, but in informal context it's totally correct to use it. We do ti all the time when speaking to our friends. It's important to know all the forms we use, not only the ones who are correct in standard and formal language.

First of all, this "ouais" is a slang, and the second "ouais" is totally unnecessary. You should use "oui" ! For baigner, usually it's reflexive, so se baigner, when you positively do the action of swimming. But, in the sentence, firstly, it's metaphorical, you don't actually swim, but the meaning of baigner in here means "to live", and secondly, you undergo this violent environment, you don't do the action of living like for swimming. Hope that it is clear !

qui ont grandi et toujours vécu dans le milieu...