The shop vac I have, from the eighties, has a ball that floats, and shuts off the air if the tank fills.
Do the new shop vacs have a motor that's isolated from the air flow?
Christopher A. Young Learn more about Jesus
However, many (if not most) "shop vacs" ARE specifically made to suck not only wet stuff but straight water. They do not pass intake air over the motor like many household vacuums do. (The first 50 or so shop vacs on HD's web site all say "Wet/Dry".)