You could try using a volumetric slab - I don't tend to use them much myself, because they can push the render time right through the roof, but if you Alt-click (or it might be Ctrl-click - it's one of them!) the water terrain button at the top left of the Bryce toolbar it will create a "block" of water rather than a 2D plane. You can apply a suitable water texture to this "block", which can then be stretched up out of the field of view. Then you can play around with the light settings for that lagoon or abyssal depths look. Word of warning about volumetric renders, though: I'm not kidding about the render times - they can be enormous, so if you're looking for a quick fix to your problem this may not be it. That's why I tend to avoid them (I have no patience!) IIRC, there's one volumetric-slab render in my gallery titled "Pollution". It's nothing special but it should give you a basic idea of the potential of the voluemtric slab. Hope this helps.