This may or may not work: You could apply a transperency map with a gradation form black or gray to white. You could use either a math node (P node run thruogh a math functions node)or an image map with an image created in a paint program. Use this to create a transperency that would make the water transperent at the shore and more opaque as it gets further from the shore. The color in the water will be preserved. Depending on how the water plane is UV Mapped you may not be able to use an image map to do this with out remapping it. Hope it helps