If you go for a cartoony look like in theme park world, you can pull off some reeally awesome low poly models, and I suggest you use sprites for the people, like they did in theme park world again. That will cut way down the number of polys you'll need to show on screen.
Also, I can't remember if you mentioned it anywhere, but how much micro management will you have? Will it be pretty simple like in theme park world so you can spend more time doing the fun stuff? Or will you go with the much more complex micro management of the rct games that gives you nearly full control of the entire theme park? I don't think I need to tell you which one is easier to code if that's what's improtant to you.
I like the water effect, very nice for there being no shader. Could you show some rides being loading into the editor or being built in it in your next pic?