Is my one battery boat going to start in an hour??

I just posted about what stereo equipment to buy on a budget and of course went a little over. Now that I have the stereo figured out, what about a battery setup. My finances are tapped so I need to decided if I NEED a second battery setup. I picked up 2 amps, 1500w total. I plan on listening to the stereo on the beach for a hour at a time before getting back on the boat. The stereo won't be fully cranked because fighting stereos with everyone is annoying. SOOO, is my one battery boat going to start in an hour?? I was thinking as a backup, I could carry on board one of those cheap battery jumpers in case the boat would not start. I would just need to make sure that the cheap battery jumper is charged every night. Also, will that be really hard on my battery killing its life span?