I also want a real damn Zora water temple and water gameplay with unlimited swim stamina/underwater swimming. The waterfall feature of the tunic is boring.
I have an irrational fear of water in video games so I'm fine with how this was handled lol.
Maybe water traversal underwater wasn't implemented because swimming anywhere you want underwater doesn't have the same impact as climbing and navigating through land? You don't have the effects of gravity and paragliding, or the sense of escalation? Of course, there can be design around that, but I can see why maybe that was skipped upon for now.