The reflective shader is very unreliable for transparency on my computer and lots of other people's, too, this is something we found out in Marc's older glass thread.
He was able to easily create a reflective shader and simply use transparency with the slider. Whereas, I had to keep reloading the scene, multiple times over and then, maybe just once after a few reloads, the reflective shader would become transparent.
Ulco, as a test, try and make a new .tgd with reflective shader transparency set to 1, save it, then keep hitting 'revert to saved'. I'll bet one time out of ten that it will load with actual transparency. It's better to use a water shader if you usually see a black reflective shader with transparency.
Other people were managing fine with just using the slider, Marc was, and I remember Walli using it before as well with no problems. Weird.