underwater hunting was not a mistake, it was genuinely one of the coolest things the series has done. the extra dimensionality it adds to a game based heavily around positioning opens things up and adds complexity, in a way that even post-4th-gen verticality can't quite match. and yes, your controls underwater are way more sluggish and clunky and a bit shit, and That's The Point
it's meant to take you (literally) out of your element, put you in a situation where the monster distinctly has the home field advantage and you have to deal with the struggle of fighting something that is far more at home and agile in that environment. fighting something like lagiacrus in the water really hammers home the feeling that this is its turf you're intruding on. then when the monster leaps back to shore you let out a sigh of relief because you get to return to your comfort zone, and when it darts to the waterline to dive back in you get that twinge of dread as you prepare to follow it
it re-creates the feeling of being new to monhun, floundering against a foe that feels far out of your weight class and is dancing circles around you. it's a feeling that no MH game since has quite replicated. and it adds a new vector for personal improvement, a new set of skills to learn and master, which to me is kind of what these games are About at their base underneath everything else
having something that so fundamentally alters the very core gameplay and creates these kinds of scenarios and emotions that no other monhun does, and the fact that you're always switching back and forth fluidly between those modes on an aquatic hunt, is just so cool and it's a concept that i think warrants revisiting