I'm currently working on a large outdoor level for my flying game, it uses a large terrain and 4 large prefabs for water, I want to know if you guys have any tips for increasing my frame rate for an area such as this.

Just in case you haven't seen it, there's an overview on optimizing framerate and a list of tips in the manual that you might find helpful (I did).

You'l find it under Level Editing/Framerate.

The terrain takes most computer power,
try to make it not more detailed then
35x35,
The number of polys increases quatratically with
the resolution, so a 70x70 terrain uses 4 times as
many polys.

You could try to split the terrain in 4 or more Parts
and switch the terrains invisible when the player is
far enough,

You could also make a lod for mapentities.

You cam use fog and use clip_far;

And you can create and remove such entities which can´t be seen. (By a script)
Look at AUM 44 for details.

did that. thanks anyhow. no more problems with frame rate. got-r-done.