If we go with your theory, why don't the great lakes have tides then?
The Great Lakes do have tides, but they are many orders of magnitude smaller then the tides in the ocean. Exactly what you would expect if tides were caused by the gravitational pull from the moon.
This is because tidal forces in the Earth-Moon system are dependent on the difference in the apparent gravitational force from the moon
over the width of the object in question. There are tidal forces on any body (water or otherwise) with two points at different distances from the moon, but the tidal forces are understandably greater with larger bodies.
The oceans have large tides because they are a very large body of water, and the gravitational force from the moon on the near side as compared to the far side of the Earth is significantly different. The Great Lakes, on the other hand, are much smaller, and understandably experience smaller tidal forces.