He regrets having stepped into that mess back then.
Tips
memory
Picture wading (趟) into muddy water (浑水) - you can't see what's at the bottom and you'll come out filthy. Same logic as English "wading into murky waters."
usage
Almost always used in the negative: 不趟浑水 ("I'm not touching this"). The colloquial advisory phrase 别趟这浑水 = "don't get involved in this mess."
