A site's bottom composition determines the life you're likely to see and the visibility you can expect. Most dive sites feature bottoms that are composed of rock, gravel, sand, silt, or mud. In general, rock and gravel bottoms feature more life and better visibility. Sand, silt, and mud bottoms are easily stirred up into the water, and are populated by life forms that spend most of their time buried deep under the floor.

The best indicator of a site's bottom composition is the composition of the nearby shore. If a shore consists of a wide, sandy beach, the same features are likely to continue underwater. On the other hand, rocky sites are more likely to be found off rocky shores.