One point about this; we believe it's already happened.
Our current theories about how a moon as large as ours (as a proportion to the Earth's mass) formed and why it's so close is that it was formed from a planetary collision.
The thinking is that a planet around the size of Mars collided with the proto-Earth early in the formation of both. This caused a massive amount of debris to be flung into space, although the net mass of the Earth increased. Gravity being what it is, the debris forms a ring around the Earth, which in the space of a few thousand years, forms the Moon.
Thing is, the early Moon was very close to the Earth. We know this because the Moon is actually drifting away from us ever so slightly. It's believed that the early Moon would have orbited the Earth every 35 hrs or so, and caused MASSIVE tides and storm fronts (assuming that the water fell back down to Earth or was condensed back after flash evaporation) because of being so close.
It's not until the Moon recedes a little that things become sufficiently stable on Earth to sustain the first life.
Based on our projections, it's believed that the moon will eventually free itself from Earth orbit and drift away as a rogue. This will cause problems for the habitability of the Earth as many of the environmental cycles we take for granted (including our stable rotation) are based on having the moon in orbit. Th the time this happens though (in about a billion years), the Earth is pretty much uninhabitable anyway because of the Sun and its increasing temperature and diameter.
The real point here is that there's information out there about the effects that the Earth would have likely experienced during the early days of the moon which might help you extrapolate this out in terms of tides and the like.