How do I design a new creature that lives in the water?

Use Tynker's mobs editor! Of course, you can change how an animal looks… But also how the animal behaves, using code blocks. Choose a mob to start.

https://www.tynker.com/minecraft/mobs/

Click on the code icon in the upper right corner to edit mob behaviors. If the code icon isn't shown, editing mob behaviors may not be available for that specific mob. Try a different mob instead.

Hint: The starting Mob you choose matters, especially if you're changing the 3D model. Choose a similar shape and body type to what you have in mind, if one is available. For example, if you have something aquatic in mind, you might start with Salmon or Squid. This will ensure that the new mob animates well within the Minecraft world. (If you make major changes to the structure of the starting model, you will likely see unexpected mob animations.) This custom shark, for example, is based on a fish with a very similar body type.

Need tutorials on how you can create custom Mob Behaviors? Check out the Add-On tutorials in your Dashboard (Project > Create New Project > Minecraft > Add-Ons) to see the possibilities.