I do feel that this is something of a valid criticism, although like many valid criticisms the point is lost in the ridiculousness of it considering it was bloody IGN that made it, and IGN have even less credibility than most. The problem with Gen 3's water routes is that they're basically identical in structure, and land-based routes usually had alternate paths you could take, and had more visual variety...which goes a long way in a game that has you traipsing down linear pathways. Dive was a nice enough idea but it would have been so much better if they had done more with it. It feels more linear and repetitive than other generations and whilst it isn't in reality, the illusion of freedom through variety has always been one of Pokemon's greatest strengths in game design - you feel like you're doing something different because you're exploring somewhere else, even though the new place is basically the same as the old place with a new coat of paint. Gen III just used the same blue coat of paint for 90% of the routes. It didn't help that there wasn't any real variety in the Pokemon available on these routes either.
It's not as damning as the godawful trumpets, but the lack of variety in scenery is one of Gen 3's biggest flaws - hence, too much water. Or rather, not enough done with it. There is more than one type of water, and the seabed doesn't look all the same no matter where you go. Perhaps you could forgive RSE for being limited in this way due to the technology of the time, but ORAS? They could have and should have done so much more with it. How about some tropical beaches? Coral reefs? Dark depths lit up by Chinchou/Lantern/Finneon/Lumineon/whatever else? More underwater caves and grottos? A completely sunken ship (think Sea Mauville but entirely underwater and with more puzzles)? Atlantis? Something, ANYTHING. It wouldn't have killed them.