Considering Tiamat - yes, she would be a fitting Pretender (or unique summon in any case)... but this wouldn't fix the general problem. Storm demons may be better - especially as there is no Air+Blood summonable commanders. But here we should also the way to summon them, i.e., Blood mages for Oceania to get (and it can't blood hunt uder the waves in any case).
Considering animals' low MR - at least Orcas could well have MR on par with humans - they are very clever guys! The same could go for Kraken mages... And corall-based monsters can have higher MR for being decentralized organisms with just no central brain/soul to affect with spells/mind blasts/dominations.
About water nymphs - yes, there were a lot of them. We also shouldn't forget Nerejos himself - father of one kind of those - who was a shapeshifter and almost could wrestle Heracles to stalemate. And Achilles himself was a sone of one of water nymphs!
So it seems we can have an opportunity to include some real thugs here...