I had two Fighter/Clerics (lvl 11/10 both) and a Paladin (lvl 11) doing Turn Undead and nothing happened. So I run for it when I see Drowned Dead coming.
I did find that casting Web and then picking them off with good arrows (I used acid and fire arrows) from a distance worked, but I used up a lot of arrows. I tried fireballs, but you gotta use too many to be of use.
That brings me to another point.
Off topic: IS THERE A STORE IN LONELYWOOD??? I found that guy at the inn, but I need more than what he sells since all my stuff I brought with me from the regular IWD game I completed isn't going to last forever in the expansion.