Lucy was the first to see them, and was curious about them. She had called Edmund over to look at them, but when Drinian saw them talking about them, he told them to come away. He warned them that they were dangerous, claiming that the females made men fall in love with them, or in love with the under-sea kingdom itself, and made them jump overboard.
Whether this was true or not is unclear, but Edmund did say that the Sea People looked fierce, and that they looked as if they would have attacked the Dawn Treader if they could had.
The ones that Lucy saw, she guessed they had to be some kind of Lords or Nobles, because of their appearance. One of them was definitely a king, as he couldn't be mistaken for anything else, who looked proudly and fiercely at Lucy, shaking his spear at her, with his knights doing the same.
They had appeared to be hunting, using some larger fish to hunt the smaller ones, like how the Nobles in Narnia hunted with hawks.
The Sea People at first looked like ordinary people, not like Merpeople, yet they lived underwater. They had legs, but travelled under the water on giant sea-horses, wearing coronets of some kind, chains of pearls, but no other clothes. Their bodies were the colour of old ivory, and their hair was dark purple.
Apparently, the Sea People were afraid of three beasts of the sea: -