I tried to do a little (web) research on this. It is of course the reason a prism separates white light into its components. I didn't find out much about sea water, though.

And then there's "slow glass", in which the passage of light through half an inch of glass takes years; the subject of the short story "Light of Other Days" :).

One of my favourite stories, heartbreaking though it is.

Arthur C. Clarke and Stephen Baxter wrote a novel of with the same title, but the two stories have nothing in common. It's worth a read in these surveillance heavy times.