For those that live under a rock, Pokémon heavily relies on a weakness/strength system based on 'types'. Both the Pokémon and individual moves have types. Hitting weaknesses will wreck faces, while hitting strengths is practically useless. This is an important preface to my point.
In the regular land terrain, you can find Pokémon of pretty much all types, which forces you to change up your own Pokémon to adapt.
In water terrain though, the Pokémon you'll find, both in the wild and on trainers, is 99% water as a main type, and it is here where we come across the real problem.
Without any grinding, you can absolutely blitz through any challenges in those areas with a few reliable Electric or Grass types or even moves, to the point where it's just not fun to do.
But at the same time, you have to go through these areas to progress, and the game heavily encourages you to use Pokémon/moves that hit weaknesses. It's been teaching you to do this the entire time. which means most players will experience the drag and not set their own fun to counteract this. That is a legit negative.