Transparancy problem

I'm still working on my swim lab age and today added some caustics to the water. The caustics image has white caustic swirls on it and surrounding that a transparant blue layer. It's a png file.
Now at one time during this day everything looked cool. Nice white swirls in my clear blue water layer. But after some strange stuff (suddenly the layer wasn't transparant any more) the color of the swirls is grayish and the entire layer looks darker (have a look at the picture). Honestly it looked so much fresher before. What I tried is set every available color for this material on white and at eventually repainted the vertex color all white. Still no change.
Btw... the water mesh is set to Shadeless. If I let the light influence the layer all transparancy disappears!!!
Any experiences with transparancy? Tips are welcome
I also included an image of the texture itself in Photoshop.
Now at one time during this day everything looked cool. Nice white swirls in my clear blue water layer. But after some strange stuff (suddenly the layer wasn't transparant any more) the color of the swirls is grayish and the entire layer looks darker (have a look at the picture). Honestly it looked so much fresher before. What I tried is set every available color for this material on white and at eventually repainted the vertex color all white. Still no change.
Btw... the water mesh is set to Shadeless. If I let the light influence the layer all transparancy disappears!!!
Any experiences with transparancy? Tips are welcome
