Hmm, the problem is that most of the tamed creatures already belong to the underwater category (such as mermaids, krakens, and naiads). Here, we could possibly include fishermen (analogous to peasants – peaceful inhabitants of pirate islands), various types of pirates, harpooners (a unit with a special ability – a giant harpoon), nixies (fish-like people, similar to those in Pirates of the Caribbean or elementals), and fishing wanderers (giants wandering on the water and catching fish with giant nets).