1)Mermaid-Undine (upgraded archer)
2)Triton-Temple Warrior (infantry)
3)Siren-Devilish Siren (flyer)
4)Huge Crab-Armored Crab (infantry)
5)Octopus-Kraken (infantry)
6)Hydromancer-Oracle (very cool archer)
7)Sea Serpent-Deep Wyrm (infantry upgraded caster)
8)- This one is a secret for now

Hmm... doesn't it seem like Krakens and mermaids, as well as tritons and other aquatic beasts, like sea serpents, are something that "remotely resembles" Disciples2? ;)
But overall the idea is good. How many arrows will the mermaid have in reserve? And will the oracle attack with arrows or lightning? What is his source?