Here is a map showing the area covered by water for US Census Tracts. Each dot represents the surface area of 20 Olympic-sized swimming pools (because I, like most people probably, suck at visualizing/comprehending big square meter numbers).
Here is a simple how-to, including a poignant letter penned to my old friend, Dot Density.
Happy Dot Mapping! John
P.S. Wondering what the same map would look like but showing the area of land instead of the area of water? My equal-area projection ensures it’s a rather boring, but nerd-interesting, map: