This list based on 30 minutes playing with your map and this mod.

specially made creatures for this underwater world.

* WaterGolem
* Archelon (turtle)
* Protosphyraema
* Pliosaurus
* Xiphactinus (Piranha)
* Geosaurus

Underwater version of (all versions schould be also tameable and breedable):
Sarco, Sucho, Achatina, Arthropluera, Baryonyx, Beelzebufo, Carbonemys, Deathworm, Diplocaulus, Giant Bee, Araeno, Otter, Phoenix (water based), Spinosaur, Titanoboa, Wyvern (water based - maybe spiting hot water), Pulmonoscorpius, Kairuku

tamable, breedable and bigger version of:
Cnidaria
Coelacanth
Manta
Megalodon
Mosasaurus
Otter
Piranha
Plesiosaur
Trilobite
Tusoteuthis
Ammonite
Sabertooth Salmon
Pulmonoscorpius
Leech
Ichthy
Electrophorus
Dunkleosteus
Basilosaurus

tamable, breedable and smaller version of:
Leedsichthys
Liopleurodon
Mosasaurus
Tusoteuthis
Plesiosaur
Dunkleosteus
Basilosaurus

specially made tiers that look more fitting inside an underwater world.

* for example the thatch skelp things look like made of seaweed and skelp and not thatchy
* higher tiers in underwater resources

Maybe disadvantages when you are on land. (reverse breathing air or faster dehydration)

New weapons and armors that look different and aore fitting more inside this.

Some Fences that shield your base even from top enemies and hold your creatues inside, like a big aviarium

Wow, lots of detail! A lot of that is in the current plan, but more ideas are always welcome!

I'm working on getting the content out first, then I'm going to go back in and fix the "appearance" of all the kelp/thatch and coral/wood structures to make them fit a bit better. There have a been a lot of different suggestions in terms of dinos to add. I'll add those to the list! Underwater golem sounds interesting... I'll definitely see what I can do with that.

If you have discord, head on over, we've got several people that are discussing ideas over there. The more the merrier!
https://discord.gg/JSA7mYQ

I will look forward how those two mods will proceed.
The problem for you is Ark doesn't have much underwater things in common so you need to remake nearly Ark completely.^^

You also need some easy mode water creatures like dodo, moschops, lystrosaurus that will give you meat, maybe hide, bones (a house that uses bones would be funny for an iron tier that you get from killing bigger creature at shark and higher), an ability to maybe create plasma lights, plasma based energy generators.

Higher stacks for resources, dont think any stackmod will come up with that.

Tamable, breedable alpha versions.

Removing the bow, arrows, crossbow and bolts. Maybe also removing the guns. Gunpowders doesn't make any sense underwater.
Improving the harpoon with more ammunition (2 or 3 loads) and higher torpor amounts.Instead of gunpowders you could use compressed air like capsule you need instead of gunpowders and for low timers a reusable spear with a string ot it to use it faster.

Changed kibble need for each creature or an kibble converter.

Higher base swimspeed

How about being able to craft one of those handheld Propellers that Divers use to get around faster?: https://www.leisurepro.com/blog/wp-content/uploads/2011/02/SDOSE.jpg , Or even a Dive Sled like the Navy Seals use to use for resource collecting: https://tse2.mm.bing.net/th?id=OIP.ia1yRir5Td4R1uVuAZX9rwEsCB&pid=Api , https://www.bing.com/images/search?q=Seal+Delivery+Vehicle&FORM=IDINTS

Hi,
the Diver propulsion vehicle would be a nice addition.

How about Mussel instead of just spawnable pearl resources on the ground. Maybe that you can collect them like giant bees. Break the "foot" of them (with special one time items to not destroy the Mussel) and gather them, then place them inside your base you need to feed them sand and plankton/eggs. With sand they produce in a ingame day or so one pearl.

Instead of an Achatina a Conidae would be also nice. It fights you and can kill you. So you could get slime/paste and toxin from it as a base source for your torpor based ammunition. Maybe higher in the ranking then the toxin of the Cnidaria.

Instead of a Titanoboa an Eel would also be nice, maybe twice or thrice the size of the Titanoboa.

Maybe some pumps, to gather resources form the ground and inside the ground.

A structure to collect or breed plankton (like a bee hive).

Improved seeds to farm your own resources, kelp, seaweed and so on.

How would be a giant axolotl. Would be a fun signature creature :-)

someone said maybe and a tamable deathworm, maybe a Bobbit Worm, prob not tamable cuz they don't have a brain.