I'll spoil Tibs post. In order to return from Devil's Reef, you must have a different investigator at Falcon Point use the written ability (pay $2) to bring the investigator back to Innsmouth.
There are 3 ways monsters spawn in Innsmouth.
1) A gate opens and a monster appears, releasing it into the streets. Also, Monster Surges can happen making things even worse.
2) A mythos card directs you to spawn X number of monsters in a certain area.
3) Flying monsters in the "Sky" space on the Arkham Game board can also stalk investigators in street locations in Innsmouth, provided they have the lowest sneak value or are the only ones unfourtunate enough to be in a street. This technically isn't a monster spawn but does provide a way for monsters to enter Innsmouth.
Have fun "rescuing" investigators from Devil's Reef. If you start a game with Silas or Ursula you will find that you can cleverly place them so that they're very usefull in your investigations of Innsmouth.