Search techniques without divers:

Underwater survey has many applications, such as forensic research for stolen vehicles and victims of drowning.

The techniques can also be used for locating and recovery of obstacles as shipwrecks, and ammunition detection in surf zones, fords, rivers, lakes, canals, ditches…

Magnetometers or electro magnetometers can be towed or fixed on a small vessel. They can also be towed in the water. (fish or probe)

A good alternative for magnetic search is the side scan sonar. Objects however need to be laying on the seabed or above.