Considering more mythological I would sooner look at some Chinese/Japanese dragons. Though they would probably look better in nations completely based on these mythologies. Oceania is mostly based on Greek mythology and classicistic art, after all. Though here we have the possibility to include more kinds of Nymphs and their relatives (Nereios the shapechanger comes to mind...), also dolphins (and Orcas already mentioned), plus Triton shell chariots (though these maybe would look more appropriate for EA?) and some other things. For one, I would add triton shell-blowers which would act as standarts (with possible additional effects of causing fear?).
Considering Marids - I would prefer to see them in separate nation. Though to be fair, I saw mentionings of Jihn being able to swim or appearing from sea. But this, I think, would be more indication of their general power rather than specific affinity to seas...
By the way, this discussion begins to repeat itself. Maybe we should begin to take some tasks each and work on it? Namely aiming for mod which would make Oceania competitive without becoming too strong?
I can try to make Triton Chariots and shell-blowers, for example...