Pokémon games are known for at least some variety when moving from route to route. The terrain and environments tend to vary too, and you're never stuck in the same one for too long (excluding the default "grassy" path routes, of course)
So when you hit that part in Hoenn where it is NOTHING but Surfing, it can get boring really fast.
On top of it, you will have to backtrack all over the place looking for the nooks and crannies for items, and then again when you're Diving underwater.
Then the last three routes are effectively a single HUGE route that is covered in uncontrollable currents, meaning you will have to repeatedly fly back to the previous Town over and over again to get all the Items, and find the Regi cave.
The concept was fine. They tried to have a game that split between ground and sea, to emphasize Aqua and Magma's goals. Problem is, on land, there was a Volcano, plains, forests, and all kinds of cool locations. In the water half of the game, it was just... Water. There was that useless town with the Space Center, and Pacifilog Town (aka "fly back here and try again")
But that was it. It took a game that was fairly entertaining up until that point, and then threw all the variety and fun out the window.
Granted (and I don't mean to offend) if it was a childhood game, you could probably overlook this due to nostalgia. I've noticed tons of people tend to think it's not a big deal, if the games were big in their childhoods, whereas fans who grew up with Gen 1, 2, or 4, are more critical of it.
There's also this:
So if you didn't use repels, you're stuck seeing the same Pokémon over and over again, and the remakes didn't even fix this at all.
I honestly really dislike the Region in general to be honest, although this is of course, heavily opinionated. I am hoping the new games can deliver the tropical Region that the franchise deserves, and I can forget about Hoenn for good.