The benefits of unlimited flying over land content is game breaking compared to being able to traverse water on a horse.

Not really comparable. If Mythical Dine got unlimited jumps or team buffs without cooldown then sure, but running on water is just an extension of Dream Dine's desert running trait. Dream Dine doesn't have "desert stamina" or any restrictions specific to itself that bar it from running in parts of the desert, so Mythical Dine shouldn't really have any in regards to running on water.

Ya well being able to run to Moodle is a bit ridiculous.  Even if we excluded margoria, being able to run from Velia to Oquilla?  That's way too much for something that's supposed to be a "vanity upgrade" as PA put it.

Unlimited flying might be a bit far fetched, but I'd settle if they made it so that you can use fast descent once the glide is over..

water stam =/= stam 

You can easily reach Oquilla's eye, even before the update. 

The ONLY difference between having or not having water stamina: you need to pause on 3~ islands on the way.
Having water stamina doesn't STOP you from reaching places, it just DELAYS you... which is just a bad mechanic.
You just make players stand in place for 10 seconds each time staring at their screen for NO REASON.

T10 Dine can't run through Margoria Sea or Ross sea anyway. (if ppl don't know, T10 Dine immediately drops you when you enter those waters)
you can't board ships. it doesn't let you hop to them. (so previous arguments about flagging on carracks is invalid)
you can't use charge on water either. 
you can't barter with it anyway, it doesn't have slots or weight.

I still haven't seen one valid argument for why the water stamina is NEEDED.

Yes absolutley! The T10 Uni needs this kind of love!

Apples and oranges.  Running on water is not an "extension of desert running", not even close.

Dismount in water (because Mythical Dine hovers when you dismount) when someone stops to do a barter, use the ladder, climb on the ship, flag.

What I've seen from the uproar over the past few days, is that griefers use 100% of their brain power on figuring out ways to disrupt people, and they are pretty good at it.

And what's wrong with this? You COULDN'T reach those places on a horse at all before you had T10 Dine, now you can actually get there without a boat, and faster than most boats. Should all T10 Pegasus owners (myself included) complain that I can't make it to the top of a mountain without being delayed by having to jump? I couldn't get there before, at least not as fast.

Agree! Make it possible and not-frustrating to get to Padix island and Iliya island without hopping from an island to another. It's literally the only thing T10 Unicorn is useful for.

So basically, remove the water stamina bar or double the stamina. Please, MAKE SOME TESTS!!! The latest changes to water stamina didn't allow T10 Dinè to run from Velia to Iliya or from Terrmian to Padix island so basically the change was meaningless.