The Sea Serpent Area is split into two locations west of Hrodmir (here and here).
You can get there with Captain Haba. You will need Diving Equipment (i.e. either a Helmet of the Deep or a 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 first 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 Serpent 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 Updates/8.5, as Sea Serpents droped little loot whose price was stable.
Spoiler warning: Quest and/or game spoiling details follow. (Settings
: hidden content)
You will need to have completed The Hunt for the Sea Serpent Quest
in order to access these hunting grounds.
Spoiler ends here.
Creatures on second floor