It would take some serious magic to account for all kinds of factors: being able to breathe underwater being an obvious matter, of course, but there's also being able to withstand the atmospheric pressure the further down you dive, accounting for any unexpected currents so that you don't get pulled or thrown into sharp rocks, whether or not you can even see anything once you start going past where sunlight can't even reach...
It's not something I want to call entirely impossible, but it would be extremely difficult to do. Even most merfolk don't bother with attempting to dive so deep into the depths of the ocean, since darkness and atmospheric pressure can still cause harm to those unprepared.
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It's not something I want to call entirely impossible, but it would be extremely difficult to do. Even most merfolk don't bother with attempting to dive so deep into the depths of the ocean, since darkness and atmospheric pressure can still cause harm to those unprepared.