There is a clever fix water routes: Islands. Along the expansive water routes and diving spots there are islands dedicated to ALL types of pokemon, not just one type.
PLUS, this adds an opportunity to add SECRET islands to search for. Imagine if there where 17 islands scattered along all of the water routes of Hoenn, forcing the player to use a comination of Surf, Dive, and Waterfall to find all 17 of these legendary places.
And the islands themselves could be something special.
I was more talking about the lack of variety in Pokemon on the water route. It wasn't that great in comparison to how vast the ocean routes were. I liked having big water routes for once, but it did make it feel a little more like a chore at times when you realize you aren't seeing too many kinds of Pokemon. This could be the time to expand on the chances of seeing others(the encounter rate was ok).
The rock-type concern isn't too bad considering you can easily take out Wingull/Pelipper with rock moves and Tentacool with ground moves. I prefered Steven as champion over Wallace, so I don't know what to say about that.
As for the secret islands thing, minus the Waterfall use, we already have those- The islands where you can find shards to exchange for evo stones. The idea of having more of those kinds of islands 'hidden'(so we don't ruin the ocean scene) with some extra Pokes is interesting though.
Yeah, the Acro Bike had the ability to do wheelies and jump, which were required for some little side puzzles like those areas accessible by going across those beams you had to jump across. The Mach bike was more about speed and had some gears, the best allowing you to ride up those mudslide looking cliffs. You had to go back to the Mauville bike store to switch between the two bikes though.