[alexti]

I think that allowing to buy PD (tritons) in the sea provinces would do the trick. It's not that difficult to get into the water (with sacred troops or without), but it's difficult to defend those provinces, because any water mage or Mound King can easily take them from you. Buying tritons in every province just to have PD-level defense is too costly (to buy and to maintain). But if Water Cult nation could buy triton-PD their sea provinces would becomes as defended as empty land provinces.