[Uhehner]

First go to Lowes / HD and purchase a digital multimeter. Then set it to measure 20 volts DC. The good battery should read over 12 volts, almost 13. Mark this cable PORT with a SHARPIE. With the engine running it should read 13.8 volts. The dead battery should read very low volts, maybe 8 to 11 depending on how dead it is. Mark this cable STBD. There are NO lightweight batteries that will last on a boat. Buy one with the highest amp hour rating that will fit in the battery tray. Then join the US Power Squadron and take the "Marine Electronics Course".  Capt. George