You can figure out the density of an object by taking that object's mass and dividing it by its volume. Fresh water has a density of 1 (gram/cubic centimeter), while salt water is slightly more dense.
When scuba divers go underwater, they want to become neutrally buoyant, which means they are neither sinking nor floating.
Divers are able to acheive neutral buoyancy using two devices:
It is very important for a diver to reach neutral buoyancy. If they are not dense enough, they will rise back to the surface. If they are too dense, they will fall to the ocean floor, possibly crushing things like fragile coral reefs in the process.