What does not have weight but is able to let a ship sink?

At a nice sun shiny day, a man rowed with his boat to an island in the middle of a big lake. On the island he caught some fish. Then he decides to return with his rowing boat. After 10 meters of rowing he looks behind him, but to his big surprise there is no island anymore. What happend to the island?

A spy satellite circles around the earth in 1.57 hours. Can you tell within 3 seconds how much time it takes until the satellite has made 24 revolutions?