The Drowning Pool has a nice, complex plot, but I don't think I really got to know any of the characters well enough to care about what happened to them. 109 minutes might be too short to do a Ross Macdonald story justice. I read about half of his Lew Archer books back in college, including The Drowning Pool, and really enjoyed them. Macdonald's prose style was engaging and his characters had more depth than most others I've seen in the mystery genre.
It could be that nostalgia for Macdonald's books is making me give this movie a higher score than I would give it if I weren't familiar with them. Also, I do like its somewhat dated style of filmmaking. It does look pretty good and I liked seeing '70s New Orleans.
Not as good as Harper, its predecessor, but far better than Blue City, another Macdonald adaptation.