Will I need a second battery?

I just bought a new "blue top" battery for my nautique. I'm waiting on a few items to come in so I can complete my stereo install. I will have at least 2 amps, but will probably end up using 3 total. Will I need 2 batteries? I think I might know the answer to this but thought I would ask anyway. I don't want to add another one,(I'm running out of room), but if I need to then so be it.

You shouldn't need a second battery if you only run the stereo with running the boat engine. But if you plan to run the system with the engine off for long periods of time, then you might need it. Either way, it is a good idea to have two batteries. I installed a second battery a couple of years ago. I cost me some extra cash but the piece of mind knowing I will never be stranded due to dead battery is worth it.

Two batteries and a jump pack.... cuz being stranded powerless in the middle of the lake is just no fun.

I run 3 amps.. 400W for interior speakers, 500w for 2) 12" subs and 200W for 2) Infinity Tower speakers. I have on/off switches for the Tower and Sub speakers and amps. When sitting for long periods, I run just the interior amp.. I have only one battery..

I always put a second battery on my boats. I've seen too many problems (including some of my own) that would not have even been an issue with a second battery. Nothing will ruin your day like a dead battery on a perfectly good boat. And it always happens in one of two situations, when the weather is going to hell and you need to get moving, or when the weather is perfect and your bobbing around missing the best conditions while tying to get help.

I run a system very similiar to JohnE. I run two batteries and never had a problem until my alternator quit. One battery died and I just flipped the switch and motored back to the dock. Nothing quite like that piece of mind. I always carry a portable battery pack for some extra insurance and to help the guys on the lake that only have one battery

Thought I would bring this thread back to life for a couple of questions. I'm taking everyone's advice and adding a second battery. How does everyone have them wired? Meaning using something like a Perko switch or what about this 2 bank battery combiner:
http://www.westmarine.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10001&langId=-1&catalogId=10001&productId=5461&catalogId=10001&c lassNum=12106&subdeptNum=544&storeNum=9

After having two batteries in my boat, I will never go back to having just one. Like Gary said, it's worth the piece of mind. Some people run the batteries together, but I always run mine one at a time.