What makes things float in water?

Buoyancy force is what causes things to float in water. To calculate buoyancy, use Archimedes' Principle: the buoyancy force of a submerged (or partially submerged) object is the weight of the volume of water that the object displaces. In other Words, find the volume of the object that is under water, and multiply that by the density of water (1000 kg/cubic meter) and gravity (9.8 meter/sec/sec).

It is also density that makes things float in water. If the object has less density than water, it will float.