John, I submit to you that a huge sharp blade going 12m/s in murky water will be hard for marine life to see coming. contact with a ship's hull is a grazing, not a blade slicing. 12m/s is quite fast.
imagine you swimming in such water knowing there was a 12m/s blade but you couldn't tell where because visibility in the water was 50cm. you would panic and with good reason.
I think such turbines could well be killer blenders in the ocean. Birds can easily see windmills miles away, in a lot of ocean water you can't see your hand in front of you even if there was light.
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by Dan Frederiksen