I would concur with the earlier post about a magnetometer. The suggestion for a sensor from Geophex is poor for this application. I would suggest a magnetometer from Geometrics along with a copy of Maglog, also from Geometrics, and you will also need a boat of appropriate size and a GPS unit with realtime correction. This is not a cheap process. I have done similar surveys in Culebra, Puerto Rico looking for unexploded ordnance on the bottom of a bay. The water was at least 30 feet deep and UXO are a much smaller target than a cannon. You would most likely end up subcontracting the whole process to someone who does this all the time. Here is a list of approximate purchase prices.
Geometrics G-882 Marine Magnetometer ~ $60K
Geometrics Maglog data acqusition software ~ $1500
Trimble DSM-232 GPS ~ $5K
Omnistar VBS GPS correction service ~ $800 per year
Boat ~ $?