Well, Tsunami does only work near coasts at least. I don't think it should ebb nerfed that much, but a little weaker would be good. I'd probably say you should decrease the average damage instead of the damage limit. Or, you could make it a Speaker spell and give cultists something else.
I personally think that Priest spells should all require not unit the unit religion but also state religion. It seems like an exploit that you can keep converting to get all the priests and keep using their spells. I'd also rather the unit not require state religion, but only have the Cure disease and Heal spells without their state religion. (It might be appropriate for priests to have alignment requirements/restrictions too.)
If you could build prists without their state religion though, it might be best to go back to having water walking be a spell instead of a free promotion for cultists. I'm thinking that in my modmod I'll make the spell grant a temporary version of water walking, and not effect only the caster. It would probably be limited to living unitcombat_melee/disciple units (maybe archer and adept too, but definitely not mounted, beat, or animal). However, I'm waiting until someone (xienwolf) adds the ability to make promotions deny access to certain terrains (ocean) and allow it again after certain techs (probably Malevolent Designs).