My problem with underwater combat is that simply isn't as fun as fighting on land. I dislike swimming after monsters especially when they retreat back into the water.I dislike how movement is sluggish and harder to dodge attacks (without a blocking weapon). I dislike how annoying Lagiacrus is with its ability to easily stun players when it passes by you. Yes, he's not much of a threat when you've got 4 players constantly assaulting his head and back, but he's the most annoying monster to fight underwater solo. Gobul can also be a pain in the ass which is why people prefer to fight him on land by fishing him out of area 4 of Flooded Forest. Personally, it's his puffed up rolling attack and lunge bite that is a pain to dodge (except when I use the lance). Speaking of weapons, the great sword is horribly slow to maneuver underwater. I get annoyed after failing to prevent a monster from retreating to its watery lair. Hell, Ceadeus is ridiculously easy to beat, but he's not that fun to fight especially when you have to follow him through multiple lengthy underwater areas.
I absolutely love Monster Hunter, but underwater combat can get the hell out. I still question how much more improved it can get. Swimming is still boring and I'd still prefer to fight on land anyway.