What is the main source of energy for hydroelectricity?

Answer 1

Water. (There is a clue in the name).

Hydroelectricity is created by the use of the potential energy in water stored behind a dam.

Actually, water itself is not a source of energy, though the original answer is somewhat correct, that it is the potential energy of the water (due to its height) stored behind the dam, we must look at what energy source moved the water (against the force of gravity) to its height behind the dam, and that source is the radiant energy of the sun. With the exception of nuclear and geothermal, the sun provides the source of all energy used on earth.

Tidal 'energy' is mainly the result of the Moon's gravitational pull and therefore is not dependent on the Sun.