[Wrana]

I like the idea of Charybis (wandering whirlpool). Kindof like a water version of Eater of the Dead.

Iirc it wasn't very wandering. But it could be good in any case.

Global Warming spell?
All shore provinces become susceptable to hurricanes, flooding of inland water provinces, global food supplies go down, and maybe temperature shifts? National spell? global spell?

I would think it global by definition.. And probably national as well - it's not in style of most other nations...

Riotous Reefs
Blocks all ship traffic. Seagoing Nations are docked, Mercs, equipment, etc. Also increases food supplies in water provinces.

I quite like corall-based ideas, but are you sure it can be accomplished (especially as mod)?

Holy Water
Gives water a semi-blessing vs undead.
Not sure if this should be a global, national, or province or battlefield. The effect could be instant and major damage or something along the line of the thing in the game that keeps telling you that your undead units in a certain province have taken damage.

There are some similar effects. Global could be like an existing Purgatory, for example... Defense against undead could be especially good for Oceania as thay are the most common way to enter seas for other nations. On the other hand, Oceania's theme is different from Marignon - undead-burning holy water is probably more for Inquisition theme...

Coral Castle
Like the one creating the kelp castle but stronger

Yes, corals able to build something on land are quite popular in SF - and it's thematic. I would include maybe some other variations on this theme - monsters enhancing PD or just generic monsters (not necessarily immovable, though generally so), coral houses built which allow either to increase population growth or income/resources, etc., etc.

Humidity
(better name needed). Improves the abilities of amphibious while on land. Global? National? Battlefield?

I think it's more of battlefield one. Global rise in humidity should have many other effects so it should in any case be high-level game-winner costing a lot...

Grindylows
These water demons were first mentioned in British folktales in the county of Yorkshire. Parents told their children stories of grindylows to prevent them from getting in the cold water in the area. Grindylows supposedly had long fingers that would drag children into the deep.
Lowers population in coastal provinces? Could be Oceanias initial action to less the power of nations keeping it from getting onto shore.

Don't understand this one. I think it would probably cause land nation to scrap something together to beat you up, rather than have them evacuate coastal provinces... And Water spells causing harmful events already exist...