If you need to keep in contact with divers beneath the boat, this portable transceiver unit will do the job without any expensive equipment. The main portion of the unit is the size of a cell phone and it works via 33-foot transducer cable that can be dropped into the water. Approximate range 600 feet (183 m). 

A typical 9V battery will give you about 9 hours of operation with the unit. You also won’t have to worry about accidentally leaving it on since it only turns on when a headphone/microphone combo is connected. Incredibly popular for charter boats, among dive masters, and in commercial applications where surface communication is important.

Ocean Reef M-100 Portable Surface Transceiver Features:

• 33-foot transducer cable
• Main unit is the size of a cell phone
• Cost effective vs. other heavier units
• Small enough to be worn on the operator's belt
• High-performance, single-channel push-to-talk transceiver
• 9 hours of operation for a single 9V battery (Not included)
• Auto On/Off when connecting a headphone/microphone