I was meant to post here last night but my laptop died as I was doing so.
All that's really 'wrong'(not even wrong) with the .tgd you've posted is that you should probably, as Njeneb says, raise transparency to nearer 1, 1 is good, and move your camera to a looking downwards, 'through' the water, instead of across it, where the reflection will cancel out the transparency visually.
Using only the decay distance and colour controls is the cheapest way to get a transparent and clear surface. Adding volume density and colour will only add render time for you here and is unneeded if you only want clear water. 'Volume' of water is a confusing idea as well, since there is no actual volumetric water in TG just now, but if you wanted to add shadows in the water-body, or make it cloudy/coloured/textured/patterned, etc. then that's where you'd find the best use of the volume controls, not for making it appear 'clear'. Well, that's what I think, anyway.
I've only changed the subsurface tab's decay distance/colour in the water shader and dropped the volume completely. Changed the pov to look downwards and I changed some atmosphere/cloud quality samples to use RTA for a cleaner result with less samples required. I'm a big fan of RTA when you can cheat it to render better looking and faster than the default renderer does.
* Seth came across a really cool way of making an ultra-clear and luminous effect in water by using just transparency values in excess of '1' some time back, too, try transparency values of 2, 3, 5 or 10, to start with. Not natural, but fun indeed, and sometimes quite impressive.
