Most of the time, you draw an Innsmouth Look card because of an Arkham Encounter, so if you get turned into a Deep One there's no big deal.
However, we've found that it's quite possible to be forced to draw an IL card when you're in an Other World. There's a mythos card that does it, and at least one OW card. And, of course, it turned the poor investigator into a monster both times. So, in the case when the devoured investigator was in an other world, what do you do with the Deep One?
Leaving it in the OW space seems completely wrong; there's no precedent for allowing monsters on those spaces. Although some of our players voted for having the monster progress along the OW track until it was spit back out of the gate!
Another possibility is that the deep one just goes straight back into the cup, like a monster encountered due to an OW card, but that doesn't seem right either.
We finally settled on putting the deep one back on the gate that the investigator originally entered. It's not a perfect solution (What if the gate isn't there any more?) but it got us through the game. Any other thoughts?