a rather interesting phenomenon have i have noticed at buttons beach at ulverstone where the old stormwater drain goes out near the hydroslide, if you go out on the rocks abit or along the old drain so you can see the waves from behind when the sea is calm and the waves are small, past sunset when the light is right the waves magically become a sort of unreal/neat/mystical black as they break and sweep forward, you have to see it to know what i am talking about !