I must agree with you, ShadowLugia, I like the games but not so much as the older ones... (I couldn't quote your post for some reason >_<)
1- I have played Emerald just once, so I don't know much about the Battle Frontier... it looks like a place where you can spend a good time and effort, but I think it's take too long to gain something intersting and useful there.
2- There are underwater areas, but yes, the wter routes are kinda boring... and there are few pokes to be found there (most places are filled with Tentacools...). Trainers usually use the same pokemons, but it's happen on trainers who are on land too. Maybe not often because there are more pokes who lives in land than on water, but it does.
3- I liked who much the graphics changed (because I never liked GSC style), but for some reason I didn't like the newer style as well... It looks way more colorful, but I don't know it could be better... as you said, water routes are boring, but who they could be different? I didn't play Diamond, but there might be some innovations which I don't know...
These are my thoughts, I liked who many things changed, but I think I'm too much exigent x) No more night graphics, different Safari Zone, didn't like pokeblocks and contests... I think I liked GSC more than any other version, so I look only for negative points on other games =(
sorry for the long post x)