You need a team of architects, or one but good just to work with you on the idea, to draw initial sketches. Then you need at least two professionals for 3d presentation, to see how your idea will look in 3d real space. Afterwards you will need previously mentioned team of architects and engineers to work out the details of the park, to set up a plan. After your park's details are worked out, everything must be drawn and written in form of architectural and engineering reports and drawings. I don't know what is your situation on issuing permits for this in your country, but that's not important now, only worth noticing. Then usually you employ a team of designers who will make everything stylish and beautiful, and again a team of 3d visualists who will make a final 3d presentation for you to see and eventually alter some aspects of the park. After everything is coordinated with architects and engineers, you can hire construction workers to build your site up, agriculturists to liven up the space with plants, and specialized crew who will build your water rides and waterfalls. That's where my field ends.
I hope I have put some light onto your project with this short info. Good luck with that!