John, there are a few different options. There is a Perko battery selector switch which has 4 positions. Off, battery 1, both and battery 2. There is also a cheaper switch if all you are looking to do is cut the power off. There are a bunch of different ones but they are called a battery disconnect switch.
My guess is that you are looking for a switch because you are running a 2 battery system to support your stereo. In this case I would not use either. In place of the battery selector switch use a Sure Power battery separator/combiner. They are right around $50 from my company, are easy to hookup and you never have to touch a friggin switch. It is automatic. Here is a link to the sure power unit. The one you would want is the 1314 which gives charging priority to the starting battery so you never are left without being able to start your boat.
http://surepower.com/pdf/separatorinterconnect.pdf