[Agema]

Well, Evo is badly hampered underwater. Nature, Air, Fire and Earth are almost non-existent for long-range damage, and the few spells that are still castable target few units or have low range.

Three paths in my view retain good ranged damage functionality underwater - Water, Astral, Death. Any underwater nation you face has at least one of those in plentiful quantities. And unless you're dripping water gems like a Jotun's sweat in Abyssian domination, supplying enough waterbreathing is not a given, especially if you then plan to summon a load of krakens, sea serpents and so on. Death is possibly your best bet, as so many undead mages are poor amphibians already.

On that basis, what you may be best off doing is maybe a relatively small force of powerful mages hitting big unit buffs.