Stereo Drained Battery when not in use???

Hey all, question. about a month ago i had my system installed, 2 amps, 7 speakers in boat 4 on tower, dual remotes, cd changer, blocks, fuses, AND a 1 farad Capacitor. Now, the boat has not been ran since the install and now the battery is totally dead. The guy who installed said it was the capacitor that drained the battery even though the system was off. Does this sound right? He said i should use some sort of isolator. I am planning to install an Optima Blue top and an isolator switch that can switch between either battery or both. I will then start the boat with the new battery and attempt to charge the dead battery back up and then use one for starting and one for stereo, ballast pumps, etc. Does this sound like it will work? he also said that with the isolator, that when in the off position, the cap will not drain the battery anymore. Does this sound right as well? Should i just dump the cap and call it a day? Sorry for all the questions, but i need help. thanks in advance. The system is only pushing 1600 watts max so approx: 800RMS. Do ineed a 2nd Battery? or just get rid of the cap, or both?