Yep. njeneb's right. Create a sphere, then put it at 0, -6.378e+006, 0. Set the radius to 6.378e+006 and then tweak it to get the right size lake or ocean. Replace the Default shader with a Water shader (don't change any water settings unless you need to). That's how people do it on their TG2 planets if they want planetary-scale oceans, but it also works for smaller bodies of water.