I also only need Squallslither to finish the Hunter achievement. I spent 3-4 hours clearing everything swimming, and only got the named rats. I started tracking at that point, killing every snake in the zone until there were no more snakes, and 30 minutes later there were still no snakes in the zone. Still 3 undertow skeletons and 7 of the various rats, about what was there before, so I believe that the mobs can respawn as goblins also. Back to killing everything, I think.
Edit: It seems like Dagnor's has a long respawn time. The snakes all respawned shortly after. Camped more for awhile and eventually Squallslither did spawn. At this point I think it's safe to say his PH is always a snake.
Edited, May 27th 2022 1:54pm by zyskyn