Basically, one could even do the reflection part as a very simple post-process only. Water reflections are rather distored, so as long as what you see on the water looks remotely like a wobbly streak roughly the color of something far away, you will probably believe it. Only for refractions you really need a separate map.
Cube maps, like I said, sure they are "more correct", but who cares.
Stuff like this has been around written in Java (and later on in Flash) for about 15 years, and it has of course forever been a functionality of major photo retouche programs. And although this postprocess-only technique is as pathetic as it can be (not even 3D!!!), it still looks quite convincing. You really have to look twice to notice that something is not 100% correct. And if the person doing the retouche has a little bit of artistic skill (well-chosen motif, placing waterline correctly), you don't have a chance at all. Like, virtually every photo in a typical glossy magazine.
Now when you use such a technique on an actual, displaced water surface, show me someone who will really be able to tell.
.... which reminds me of an awesome game from 1987. Sure enough these are not real reflections, but you get the idea. The sunset looks very realistic even with only 16 colors (or was it 256?) and a resolution of something like 320x200 back then. All you see is a wiggly smear that has approximately the same color as the sun, but that's good enough to be convincing.