Underwater metal detectors are a real must for small area searching.  There are many available on the market today and there are many factors to take into consideration and the first is to decide what you want to look for and what type of coil you will need to perform that task.

Searchcoils come in two types: Concentric and Widescan. Concentric are preferred for discriminating. Widescan are less affected by ground mineralization. All searchcoils are electrostatically shielded and waterproof.

In general, larger searchcoils will give better depth with larger targets and better ground coverage. However, sensitivity to smaller objects may be sacrificed. Smaller searchcoils will offer greater sensitivity to smaller objects and improved target separation in trashy soil.

A third style, the Printed Spiral, is made only by Tesoro for use with pulse induction circuitry. It offers good sensitivity to a broader range of target sizes and improved sensitivity to less conductive targets such as fine gold chains. All searchcoils are electrostatically shielded and waterproof.

Make sure that when you are using your metal detector on beaches ensure that you have the necessary permissions to do so or you may end up with your equipment being confiscated.  There are many clubs and magazines out there to help with all of your searching and last but not least, good hunting.