As MA Pelagia you have 4 different amphibious mages. Pelagian Mermages which are W1 + 100% AWEN, Pelagian Mystics which are 4 CP and A1W1E1 + 100% AWES, Conquerors of the Closed Realm which are capital-only and H1 + 100% AES and finally Daduchoses which are coastal only, 4 CP yet only F1 + 100% AWES. Since you have access to Air and Water, you can also forge amulets of the Fish to get some of your aquatic mages onto land too, though obviously not in very large numbers. You also have Summon Hekateride and Daktyl at Conj 5 and 6 but unless you want high Nature or certain magic items forged I wouldn't bother with them. Troop-wise you have unarmoured mermen with coral spears and nets, medium-armoured wave warriors with tridents, Champions of the Closed Realm which are well-armoured, cap-only sacreds, heavily armoured mermen hoplites on the coast and semi-elite Apostates of the Closed Realm on land wherever there's a temple.
Personally I would recruit a fuckton of Pelagian Mystics and forge amulets for your Titron Kings. Mermages are rather lackluster, Conquerors are only super useful if they roll Astral, and you probably don't have the gems to worry about your other aquatic mages. Titron Kings cast huge water spells and maybe lead communions, Mystics throw out lightning, E3 buffs or Frost Bolts depending on crosspaths, and you use your wave warriors and Champions of the Closed Realm to help you get a foothold on land. On land you're recruiting as many Mystics as you can simply because you don't have any other option. If they have good paths then they can throw out some acid spells or act as communion slaves. The addition of mermen hoplites to your army is great since their heavy armour and long spears can hold the line while your wave warriors and Champions flank. Apostates are just okay. They're flexible but you'll probably want to focus on your more specialized units instead of them. Similarly the regular mermen are pretty useless unless you really have some high defence units you want to trip up with nets.
To summarize: Built a ton of forts underwater and spam Mystics, wave warriors, Champions and eventually hoplites. That is the core of your armies, to which you can splash on other mages/troops as needed or desired.