It's nice to have a set of games that have a more vast ocean to explore, all land gets stale. That's all we've basically had aside from Gen III.
Making more of the previously inaccessible islands have something going on on them is interesting though.
Rock pokemon are near unusable in Emerald. The 8th Gym, Champion, and the routes leading up to them are filled with water pokemon.
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.