Here are some prehistoric sea creatures I would love to see in this mod:
Mosasaurus: I know this is already in the mod, but I think it should be a bit bigger, because the mosasaurus from the mod doesn't look like 18 blocks. I also think that aswell as the blue and green variants, there should be a cyan variant, similar to the Mosasaurus in Jurassic World
Megalodon: There have been lots of shark models in many mods, but very few of them look realistic. This is because of the way their snout slopes inward. I am not saying to copy this model, but I suggest you look at the WildCraft great white shark for inspiration. Make it longer and bigger, then make your own improvements, and you have your megalodon.
Fish can be hard to create in Minecraft, but I think the PaleoCraft dunkleosteus is a good example. I suggest basing it off that one, maybe making a few tweaks here and there, and there you go.
This is a more little known creature. I think that it should be similar to the mosasaurus and liopleurodon, but with longer front flippers and a neck. In terms of colouring, I think it should be purple with a cream underbelly.
The styxosaurus is a large plesiosaur that I think should be in the mod. I would recommend it being longer than a plesiosaur, and green with a thin neck and narrow head.
This one is easy. Slightly bigger than a nautilus, with a brown shell and golden brown body.
Dakosaurus would be a small crocodile-like creature, with a shorter nose than a crocodilian. It would be like a slender, smaller mosasaurus, with greenish-gold skin and a sail down its back.
A nightmarish pliosaur from the Jurassic, this one would make a grisly addition to the mod. I think it should be slightly smaller than a mosasaurus with dark grey skinning and red eyes.
This is another easy one. About one block long and 0.5 tall, this small turtle would be brownish grey with a golden brown shell.
Since I know this mod likes basing their models of BBC reconstructions, I think this large fish should look like it did in Walking With Dinosaurs: Sea Monsters.
I know this is in the mod aswell, but I think it needs improving. It should be more slender, with a thin neck and narrow head. I think it should be navy and dark blue.
I would like Baculites to look like it did in the National Geographic special Sea Monsters, with a dark grey shell and bright orange skin.
The Geosaurus, in my opinion, should be based off the Ludia app Jurassic World The Game version of Geosaurus.
Helicoprion should be like a much smaller Megalodon, with a longer nose and the famous whorl of teeth.
Again, I know this is in the mod, but I believe it should be slightly bigger. Otherwise, it's perfect.