In my version, OO will probably become weaker in the early to mid game, but will be awesome in the mid to late game. (Not all of these will be read soon.)
In my version no basic priest requires its state religion, but there spells do. A water walking priest anyone can build is too much, so Cultists are losing the ability. Tsunami also seems a little overpowered,
so I'll make it even stronger ( adding a chance to temporarily change land to water) and move it to Water III.
Cultists will probably again have summon Floating Eye (Metamagic I will instead create a temporary/requires-caster Ring of Warding building, which gives 20% magic resistance to units in the city), and something else (I'm not sure what yet)
I just got to thinking that it would be really cool to add lots of INVISIBLE_SUBMERGED sea monsters that, like land animals, can't enter national border while under barbarian control. I would then let only OO UUs see INVISIBLE_SUBMERGED units, and give OO priests a spell ("Call to the Deep Ones"?) that, like Mesmerize, gives them a chance to capture these units without combat but still based on relative strength. I might remove Krakens as a summon, but have lots of them out at sea for OO priests to try to capture and use against enemies. Cultists would be able to do it, but Speakers would have a much better change of success (and wouldn't need a ship to go and and find the beasts). I'm thinking failure might cause the caster to become crazed...and success may leave the monster crazed, so you won't keep control for all that long.
Drowns will be unbuildable, and can't upgrade. They are only created by sacrificing a living (non-immortal) melee unit, and will have strength based on that units strength. (Drowned Phalanxes are quite nice, as are drowned Heroes.) (I've been making the spell work like this since 0.20, but the functions I had to use were changed in the BtS conversion. Before that, I let them upgrade and they held on to their extra strength. I could sacrifice Losha for a stronger Drown, then upgrade it to an insanely strong Eidolon. Without checking for the immortal promotion, I could do this every turn (well, most had to stay Stygian Guards). After the conversion the extra strength doesn't carry over. I removed upgrading because I think the AI was upgrading str 20 units to str 6 units, or something like that.)
Drowns, Stygian Guards, and Saverous will have water walking, but be unable to enter Oceans
until Malevolent Designs
(I think I'll also make it so that when Valin kills Saverous his owner gets a Saverous_redeemed unit, which is basically an oversized warrior with Strong. This is mostly for flavor)
Speakers will have a spell that grants a
temporary water walking promotion to their whole stack.
(The starting settler promotion will allow water walking too, so you could position your capital on an island f you plan to be OO if you like.)
The New Coastal Coral Feature will be impassible to ships, but not water walking units, so they have a strategic advantage.
There will probably be some unitcombat_naval promotions only OO civs can get.
Ships in general will probably be stronger (and all be able to enter Oceans after Astronomy), but Boarding will be available to all melee units with amphibious and combat 5. Water Walking Boarding units are the best.
(One of?) The OO cathedrals/advanced temples will grant +1
on all water tiles in the city radius.