The sea serpent area is an underwater location northwest from Svargrond.
You can get there with Captain Haba. You will need a Diving Helmet (Helmet of the Deep, Depth Galea) to dive underwater; once underwater, you can remove the helmet but lacking full drown resistance will incur the usual Drown Damage.
There are 3 floors in the area. On the first floor there are only blood crabs and young sea serpents, however you can levitate or use parcels, pots or ice cubes to get down to levels -2 and -3 to fight young sea serpents and sea serpents in a dozen or so pits. Some of the pits have caves so beware that there can be more serpents present than you can see from above. It was said a profitable hunt here was hard before 8.5 update, as Sea Serpents droped little loot whose price was stable.
Creatures on second floor