This is true. Also to add to that, in Render Options, you can select to render it all, including hidden layers, or ONLY selected or active layers, or only selected objects. That would be useful, for example, when you have complex materials applied to the water, but all you want is to do a quick preview render of the objects IN the water-- you would choose not to have the water layer render, and that will speed up your render time; even "preview" mode can be slowed down considerably by complex materials. As for me, I like my previews to be as close to the finished size as I can handle, so doing such things as hiding objects or entire planes, be they ground,water, or cloud planes, makes Vue-ing much quicker and simpler.