Yesterday morning I did a 2.5-mile open-water swim on the Severn River with three friends from my Masters swimming club. It was the second time this season I went out. About three weeks ago a group of us swam, when the water and air temperature were about 54 degrees. This time, both were about 10 degrees warmer. Herald Harbor, where we swim, is a small cove a little less than a mile across. The shore is dotted with houses—many looking like they were built in the 1950s as vacation homes. Many homes have docks for recreational boats and there’s a marina on the southern end. In the cove, about a quarter mile from the shore, is an island that measures maybe 600 feet by 1,000 feet. If you go to Google Maps and type in Herald Harbor, Maryland, you’ll see it. I don’t know the name of the island. The satellite view shows four docks and several big secluded houses or compounds on it. The water to between the shore and th...