The most efficient method to spawn these monsters is to travel back and forth between ( 19, -21, -270 ) Copy/waypoint 19, -21, -270 and ( 422, -13, 27 ) Copy/waypoint 422, -13, 27, killing all "drowned" skeletons.
Drowned skeletons can also be found around ( 1181, -24, 1052 ) Copy/waypoint 1181, -24, 1052.
Note: It's a level 22 quest, but most mobs are 22-25, and hit pretty hard when you're level 22. Also quite a few elites kicking around!