I took this image from the ferry as it was arriving in port last week. While looking at it full size on the iMac I spotted what looks like a fin. Certainly not the shape of a buoy.
You may have to follow the link to see it full size on Postimage then zoom in just below the sun and right a little.
https://postimg.org/image/p4wmz8kv9/
I have seen dolphins before from the same ferry .
What do you think?