Anything that has a volume of one cubic centimeter and weighs one gram is said to have a specific gravity of one. Anything that has a volume of one cubic centimeter and weighs more than one gram is said to have a specific gravity of more than 1. Gold has a specific gravity of over 19. That is, a cubic centimeter of gold will weigh over 19 grams. Anything that has a volume of one cubic centimeter and weighs less than one gram is said to have a specific gravity of less than one.
Most types of wood have a specific gravity of less than one.
If the beaver's specific gravity is greater than one, it will sink. If it is less than one, it will float. It is that simple. Let's say that another way. A beaver's body displaces a certain amount of water.