a bad connection. You can do it by jumping each connection looking for a bad switch, terminal or crunched wire on every circuit on the boat. Each of the connections should read infinity or close to it. By that I mean almost zero ohms of resistance. When you jump a switch that is bad, some times that switch will cause the light to light or the motor to run just by changing the resistance enough to allow enough power to pass. If you are looking for current flowing from a slight ground on some item in the circuit which is manifesting itself in a low or dead battery then you would look for current flow in milliamps. You would set up your meter in the negative leg of your battery and turn off each circuit and when you get them all off you should have zero current flow. If you still have some amps flowing then you need to start seperating out sections of each circuit to try and find out which section of which circuit the current is coming from. Two or three milliamps with everything turmed off is not that bad. You have a normal build up of salt covering almost everything on your boat and some current will flow to ground over the salt and find it's way back to the negative post of the battery. Much more then that then you have something starting to short to ground. I hope this helps. I'm doing this on my phone so the details might be lacking but I'm sure you get the idea. Good luck.