I understand that stack mode is not active for CS6 Standard. It is another way to get blurry water without ND filter?
A blur filter with the water selected.
Change the blend mode of your "stacked" layers to lighten, then do a merged stamp visible ctrl/cmd-alt/opt-shift-e. Then if it need further smoothing, do as JJ said and use blur or the smudge tool.
The method worked smoothly and I carried on with JJ suggestion using smudge tool. Happy with the result.
Thank you both