Question

what is the basic cause of ocean tides

Answer 1

Gravitational pull of moon

Answer 2

The moon causes the tides. When it is overhead and when it is underneath. The Sun also causes tides, but it is a smaller effect. When the Sun and Moon line up, the tides are stronger, and when they are on opposite sides of the earth the tides tend to cancel out and are smaller.

Tides also depend a lot on the shape of the ocean floor near the shore (is it shallow or deep, etc.)