A cutoff switch is not really necessary. When the bladder has fully collapsed, the pump will just attempt to pull a vacuum, something which these little compressors are typically not very good at, and the load current draw will usually drop slightly. You can measure how long it will take to empty the bladder on the bench, and you can mentally tick off how many seconds you ran the pump when diving the boat. If you over run it a bit, there is no harm. The only risk is if there is an air leak in the plumbing. Then the pump may ingest some water. This is generally not a good thing, so you should always inspect the system for leaks prior to starting your patrol.
George "Crazy Ivan" Protchenko
“There are the assassins, the dealers in death; I am the Avenger!”-Nemo
"I'm disinclined to acquiesce to your request; means No!"-Capt.Barbossa