Well, water can be a really tricky thing here. For your case, you just want some type of lake, so particle waters are not what you want, and you don't need to worry much about the physics of water. For some lake kind of water, a plane will do just nicely. The trick is getting the material correct, that is the key here. Water is both reflective and refractive, just to let you know. It's basically transparent but you often see water as blue because of the sky. It's really all about the material in your case, so if you're good with that, work on the material, I gave you some good pointers to help you out. If you need any more help, ask
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