Depth measurement in seawater

The Cornell sub group shows a depth sensor board provides real-time information about the craft's depth below the surface of the pool. Analog pressure data (up to 100 PSI, or approximately 55 meters below the surface) is obtained from an MSI UltraStable-300 pressure transducer, operating in a 4-20 mA current loop. A 24-bit AD7791 Sigma-Delta Analog to Digital Converter is used to capture pressure data, which is converted into a depth reading by an on-board microcontroller and reported over RS232.  $100 from digi-key.  Also a microstrain meter to show position

Also see http://www.paroscientific.com/depthsensors.htm for other depth sensors

and Woods Hole Deep Submergence Lab