The ocean is a tiny fraction of the planetary mass, so it's mostly the bulk mass of the planet. If you watch the moon forming simulation posted here, you can se

The ocean is a tiny fraction of the planetary mass, so it's mostly the bulk mass of the planet. If you watch the moon forming simulation posted here, you can see right before impact the bulging of Theia toward Earth. This demonstrates the non-rigidity of planets pretty well. This happens to a much smaller extent between the moon and Earth and is the reason why the moon is tidally locked (one side always faces us) and is slowly moving away from us -- the tidal forces are transferring angular momentum from the Earth to the moon. Consequently as the moon moves farther away, the length of our day will increase.