Floating, lift and energy sources
Something floats when it pushes aside enough water for buoyancy to balance its weight, so shape matters as much as material.
A worked example
A steel nail sinks but a steel ship floats, because the hollow hull pushes aside far more water. Fill a submarine's tanks and its weight passes the buoyant force, so it sinks.
The mistake people actually make
Renewable means naturally replaced about as fast as we use it, such as wind, sunlight and moving water. Coal and natural gas took millions of years to form, so they do not count.