Hi Rudder,
No, that's not it. Consider that the orbital period is 28 days, the centre of the rotation is very close to the earth, and you'll see any centripetal force due to the Earths orbit of the moon (YES, it orbits the Moon just as surley as the Moon orbits the earth- they are a non-symetriacl binary system) is very small.
No, it's because the far side of the Earth (from the Moons POV) is sufficiently further away from the Moon than the near side for the Moons gravity to be have appreciably less effect there. This Gravity Gradient means on the near side it pulls the WATER away from the EARTH, on the far side, it pulls the EARTH away from the WATER!
PBL,
Hope bridges are mended.
Now, do you see what I mean about the Earths rotation being an absolute, as it is an acceleration?