AIS would be an easy way to track the charter boats. Not sure the charter fleets have AIS transmit capability.
VHF radio would need to be turned on for this to work. Anyone could track the boats location.

"Automatic Identification System (AIS) is a system that provides real-time information such as tracking and monitoring for ships and other marine vessels designed to improve maritime safety. This system is comprised of transponders fitted into your boat that use short wave VHF radio signals to broadcast the vessel’s position. "