Beg, borrow, steal or buy an inflatable globe or other spherical representation of the Earth that can be tossed around the classroom.
Have students throw the globe to one another around the room. When a student catches the globe, he/she should look to see if his/her left thumb is on water or land. The student will call out "land" or "water" and the teacher (or another student) keeps a tally of land and water catches on the board.
At the end of the game, analyze the data and you should find that about 70% of the time, the student's thumb was on water. A great introduction to a study of oceans and water - emphasizing the large percentage of the Earth that is covered with water.