CAPE TOWN — YOU get a good feel for the health of the oceans when you stick your head in them for four weeks. This summer, I swam long distances in the Seven Seas: the Mediterranean, Adriatic, Aegean, Black, Red, Arabian and North Seas. The longest swim was 37 miles and took me two days.

The swims were intended to draw attention to the health of the oceans. But I seriously underestimated the urgency of the issue I was swimming for. As the United Nations Patron of the Oceans, I have given many speeches stressing the need to protect our environment for the sake of our children and grandchildren. I now realize it’s not about our children. It’s about us. And the situation is much worse than I thought.

I was shocked by what I saw in the seas, and by what I didn’t see.

I saw no sharks, no whales, no dolphins. I saw no fish longer than 11 inches. The larger ones had all been fished out.

When I swam in the Aegean, the sea floor was covered with litter; I saw tires and plastic bags, bottles, cans, shoes and clothing.