It is interesting that, during the Illiad anyway, Calypso was isolated on an island just as Isma is.... or at least she seems like she has been. Those villagers almost don't seem to count as company imho. Not that I'm claiming that Miura drew inspiration from Calypso specifically.
Why do the villagers not count other than because you don't want them to? It's not like they've been transformed for ages from what we understand (probably only since the worlds merged). Isma's personality and attitude can easily be explained by the fact she's a young person who's never seen the world at large, and whether or not she's had (boring) company matters little in that regard. It's not too different from what motivated Isidro himself to leave his home and family, or what pushed Morgan to venture into the forest. Even Puck justified leaving Elfhelm for that reason.
Besides, given her looks on the last page of this episode, she might not have been on that island for very long (having spent most of her life underwater and/or in the astral world).
They also have been known to be directly linked to Sea God's... sometimes even THE Sea God. Not that I'm claiming the Sea God Miura is referring to is Poseiden, but this could've been a big inspiration for him...
This Sea God is an ancient being that sinks ships and eat humans. Or turns them into its minions. Nothing we've been told about it so far matches the description of Poseidon
at all aside from the words "sea" and "god". I mean do you really think it's going to be anthropomorphic after seeing the idol the villagers worship(ped)? And on a side note, the nereids weren't just
sometimes related to Poseidon in Greek mythology.
Anyway, I take it your point here is that she could be the sea god's daughter or something equivalent, forced to serve it but good at heart? Seems quite unlikely to me at this point.