if you are deadset against a second battery, get a good quality deep cycle, but if you are planning on parking a lot, the second battery is almost a must. my guess is that after installing 2 good quality amps that will draw a lot of current, you will be using that jumpstart. personally, the security of a second battery is worth it. getting a tow/jump sucks..

caps store energy and release it as the amps require it. your bass amp is usually the one that will draw the most current(hence the pulsing headlights in a high-wattage auto system). using deep cycle takes away some of the need, and since boats usually don't have all the other draws that a car does, caps usually are not needed.