yellowmac's got it right with wikipedia
Here is a very pronounced picture from the mast of a navy ship
http://alpenmic.com/photos/caribbean_2000/c2000-07.JPG
Remember the amount of curvature is dependant on the height, so if you are at sea level, you will not see much curve, but if you are on the cliffs above the ocean, you will see a fairly pronounced curve.
as seen here taken just above the ocean:
http://www.dlund.20m.com/images/Mol0812g.JPG