[Illuminated One]

Since I played Oceania some time ago and was utterly crushed by vfb on turn 8 or so I don't think the Ichtisaytirs are that good.
I have an sp game with a mostly similar setup but completely different indie expansion now and that worked quite well. It still has some problems, though with a high dom it works fine. If you drop only air you can get a wyrm with good dom and n5d2s3 or e3, but imo a high dom alone could be enough.

Maybe it's my inability to use them, but even groups of 10 ichtycentaurs + 2 smiters (=680 gold) are slaughtered by tougher indies and have so many losses against weaker ones that you have to reinforce them soon. And that's something that you never want to do underwater, by the time your reinforcements have reached them indie expansion is finished.

What usually happens is they build up fatigue while only killing a little. And as they do their defense drops faster than the attack of the indies and their prot, too (17 defense is really 13 defense with +4 shield parry which isn't to great against criticals).
And since you'll see very long battles that's bad.
On land they work ok, but underwater there's just so much you have to avoid (chaff, nets, Illythids).

Oceania has a quite good unit that you have overlooked I think though, the Ichtisatyrs.
They cost 8 gold 2 res, with ok stats, except low morale.
I build as much as I can on the first turn and make groups of 10 with my starting army and set them on attack rearmost, spread them in a line.
If you fight in your own dom, with sermon of courage and the tritons to boost morale they usually deal enough damage to all but the toughest indies to rout them before taking much losses themselves. Really, my first party ran out of provinces to conquer before it was useless.