Fritz Leiber treated this very nicely in his science fiction novel "The Wanderer." Basically, hyper-advanced aliens come down and eat the moon...

He noted that, in many cases, earth's tides consist of very large bands, across oceans, of standing waves. Now, odd things happen when you interrupt a standing wave. You might catch it at just the right moment to dampen it down to almost nothing... Or you might catch it during one of its self-reinforcing phases, in which case... GIGANTIC TSUNAMI!

(Serious kitchen experiment: slosh water back and forth in a cake pan, creating a standing wave. Then stop the pan cold, doing so at different times during the wave cycle. At some times, the water will just slosh to a stop...and at others it will splash right over the edge of the pan!)

In time, the earth's tides would settle down to solar tides, about 1/3 the magnitude of solar/lunar tides. It would be enough to maintain sea life, and (Leiber speculated) there wouldn't necessarily even be a large number of extinctions.

If the moon were hollow *but retained its full mass* then nothing would be different at all. If the moon were suddenly hollowed out, so that it shone just as brightly, but had very little mass, then Leiber's scenario takes place.

Silas