Nope, ring of fire can't be cast anywhere unless you are willing to either hurt your own units or declare war at other civs.
Well. Guess what: No different for Tsunami. (not that your statement is wrong. Its important to note in respect to the assesment the OP has done.

)
As it is now Maelstorm is a far more serious problem in its current form (yes its far more useful than even Tsunami is. Save perhaps Archipelago maps...

). Oh and it doesnt have smaller range than Tsunami and it isn't only limited to sea and cost.
OO is really good on Water? Guess what: Its supposed to be!

Kilmorph is good for Gold and Production, Fol is good for Elves and in Forest and so on. You can't build cultists if your State-Religion is not OO.
Oh and that the opponent could not build a navy to retaliate because of the Range is just BS (20 Ships are killing a few Cultists in a blink.). Caravel + Oarsmen + Navigation 1 = not possible to reach by Tsunami before the Caravel hits you. That counts for most ships btw. Especially if Air 1 is thrown into the mix.
Haste (which requires a Water-Walking Mage or a Boat-Going Adept / Vampire) is your only chance to even reach the Caravel with your spell! (let alone with the Cultist itself) if you are in its range. And Air 1 can not be used on Cultists... ("Only" haste can)
Not to mention an Air 2 loaded on a the Caravel as well. That whould mean cultists suck in that comparison. (And to note: They don't since its not just a 1 on 1 combat and unlike ships they can attack units on land directly. They are supposed to be good on Sea. Its just far from them being invincible or the only feature in the Game that is really strong.)
Very noteworthy is also Arcane Barges. They do the same thing to a Stack of Cultists and also costal.

+ they Bombard City defenses (to cities up to 2 Squares from the sea. (sorry had a misquote here. I belive Fireballs can't bombard anymore if run out of movement. Right?))
+ they can also units kill directly with their fireballs (unlike tsunami).
+ they can attack Stacks / City defenders and Units up to 2 Squres from the sea. (and with shipyards they are easier to mass-build as experienced versions)
+ they carry units (like Air 2 Casters which also speed up the Barges with Air 1)
And they don't require you run OO to build them.
Now if the AI should grasp that one... (and its in the works just now.)
Cultists are good against AI? Guess what: Everything is right now. AI is not done as of yet. Wait and see. When it will, might be you will long for the time it was easy prey.
Cultists are good against players? Depends on the player. Play some more and you will find a way around it or an opponent who does...
Trust me. Just because you don't have grasped everything in the game yet (and few players really have. The game is extremely diverse) and find something powerful that did help to win in your! game doesn't mean its overpowered outright. Such things might be said after! many tests in multiplayer against experienced multiplayers, which are indeed around. (and then FFH2 isn't designed mainly for multiplayer but mainly for singleplayer.)
For example: Have you tried to charm those cultists for once (then they can't attack anymore for some time)? Or even better: Entangle them with a cultist turned druid

.
Then 1. They cant cast Tsunami anymore 2. Can't move anymore 3. That cultist-druid can Tsunami them! next round. After that those poor chaps are fooder for your Caravels, Privateers and Frigates or Arcane Barges.
Oh and you only need one Druid for that. Even for a big stack of them.
That said i whould go as far as agreeing that Tsunami needs a lower damage cap (50% sounds good in my book). But so does Maelstorm (which should have 25% or 30% at the max.) and it needs to be higher than Maelstorms since it offers less utility / is more specialized.
But i think that all stack-busting spells should be a bit more limited. And thats not because Tsunami is especially outstanding in comparison.