As far as I can tell only the Gulpers, Crawlers and Anglers count towards this achievement as they are the only new sea creatures added in this DLC.
You will fight a lot of these during the story / exploration of Far Harbor so I would worry too much about farming them but if you do want to farm them the best place I found was the marsh area between Haddock Cove, the Ruined Church and Fringe Cove Docks which is located in the South West of the map. Just walk around this area hitting the VATS button (
) as it will target them when they are in ambush mode. When I did this I only needed 4 more kills which I found easily but the enemies should respawn within 24 hours if you need more.
Here is a screenshot of the area:
Thanks to Shawn1991 for the information that Crawlers count towards the achievement.