To make it a bit more concrete:
Underwater city:
Requires: Coastal city.
Requires: Fanaticism
Requires: OO state religion
+1
for sea resources.
Aegean Initiate:
Techrequirement: Fanaticism
Buildingrequirement: Underwater City
Strength 5 can use weapons
Unitminimumlevel: 1
Productioncost: As a champion
Waterwalking, Halfbreed-racialpromotion (+2 cold strength -10% graslandstrength, -10% plainsstrength, -10% tundrastrength, -10% icestrength, -10% desertstrength --> all except ocean tiles)
Aegean Guard:
Techrequirement: Fanaticism
Buildingrequirement: Underwater City
Strength 5 can use weapons
Unitminimumlevel: 4
Productioncost: 150% of a champion
Waterwalking, Pureblood-racialpromotion (+3 cold strength -20% graslandstrength, -20% plainsstrength, -20% tundrastrength, -20% icestrength, -20% desertstrength --> all except ocean and coastal tiles)
Aegean Mystic:
Techrequirement: Fanaticism
Buildingrequirement: Underwater City
Strength 5 can use weapons
Unitminimumlevel: 6
Productioncost: 200% of a champion
Waterwalking, Illithid-racialpromotion (+4 cold strength, +20% chance of gaining control over defeated units -30% graslandstrength, -30% plainsstrength, -30% tundrastrength, -30% icestrength, -30% desertstrength --> all except ocean tiles)