There are currently no tameable aquatic mobs (or somewhat tameable aquatic mobs) in Minecraft, so I think this should be fixed by adding seals or sea lions... or even both!
Seals would be passive mobs that spawns naturally in both beaches and cold biomes such as ice lakes. They would be hostile towards salmon and cod, and you can tame them (or at least somewhat tame them) by feeding them raw cod. They would help protect you against hostile aquatic mobs such as guardians and drowneds.
Sea lions would be neutral mobs that spawns naturally in beaches across the Minecraft overworld except in icy beaches. They would be hostile towards salmon, cod, and tropical fish, and could be (somewhat) tamed by feeding them raw salmon and could help beginners retrieve fish of any kind (except pufferfish) to help give players food.
Here's my ideas for these marine mammals in Minecraft. I hope you liked this.