Here are several versions of the same simple water environment each decaled with a different sky image on a 100% ambient sky dome. The water surface is a 20x20 patch grid with displacement map, surface set to 90% reflective, black base colour and with a subtle light blue-gray camera fog. The displacement map is a single non-repeating image at 2000x2000 pixels, this removes problems with seams appearing between smaller repeating manually tiled decals but makes animating the scene problematic due to memory requirements.
This technique, though still valid, has been much improved in recent years. The bump and displacement map generation can now be easily done within Animation:Master and extra details such as varying the reflective falloff with gradient materials help add further realism (if that's what you want). I'll try and make an update of this study soon but you should take a look at the excellent water article by Charles Babbage.