A hydro vent was a type of technology located on refitted Constitution-class vessels.
In 2287, Captain James T. Kirk asked Montgomery Scott how to get to the USS Enterprise-A's turboshaft. Scott told Kirk, Captain Spock and Dr. Leonard McCoy to head down a tunnel and turn right at hydro vent, near the blowscreen, after that they would find the turboshaft. (Star Trek V: The Final Frontier)