tides

Two things mainly; the gravity of the sun (to a lesser degree), and the gravity of the moon (mainly). Without the sun and the moon, there would be no waves of the sea, and all aquatic life would die for lack of oxygen because the waves of the sea take oxygen back under the water with them, just as an oxygen pump does in a fish tank.

I am no scientist by any means, but I think that they are caused by the moons gravitational pull on the earth, or something like that.

easy xxxxxxxxxxxxxxxxx earths magnetic polarity and the moons magnetic pull and gravity working together

Some people think there are giant schools of small fish swimming all together in the middle of the ocean, being pursued by giant predator fish in even deeper water below them. As these fish get near the shallows of one end of the ocean they suddenly turn and go the other way. This constant back and forth synchronous motion of the swimming fish causes the tides. At least that is what some people think. Of course, people who believe this are unbelievably ignorant and superstitious. The gravitation pull of the moon as it orbits the earth causes the tides.