The Neon Rain by James Lee Burke (1987) Henry Holt & Co.

Dave Robicheaux, a Lieutenant in the New Orleans Police department struggles with the death of a young woman in another Parrish that is written-off as a drowning death.  He is sure it was a murder and checking into the case causes a lot of bad guys to come out of the woodwork - even his partner.  When his brother gets shot, he goes after the mobster who was supposed to be his friend.