[LDiCesare]

You can take a W9 bless for your knights to begin with. They can go out of the water, so they are very much worth it. No use to give them Nature since they have recuperation if I'm right.
You don't really want a rainbow pretender because Voice of Tiamat will find most of what you want underwater, so the only path you may want to take for site searching would be astral, but if you face R'lyeh, it may not be a good idea to put anything but a lot of astral on your pretender.
I'd pick something like one of the amphibious titans with Water 9 and 4 or 5 in Earth (since your mages lack this path) or 6 Astral. Add a good order scale to buy your sacreds (capital only, but strong), and maybe growth to avoid your bishop fishes from dying too fast.