The thing with these archaeocetes is that they do not spawn at shallow waters yet not very deep either. And to make things worse, they can attract harems of hostile manta rays and ichthyosaurs to "feed on its leftovers" as mentioned in the dossier

Overall, make sure to bring one of your powerful water mounts to the spot, as you'll have to fight those off

They provide shock and temperature resistance, yet take damage at certain altitudes below the water