People run a single or even two amps with one battery all the time. Unless you're the type to sit with the engine off and play the radio for hours then you don't necessarily need a second battery. A regular cranking battery isn't ideal but will be ok as long as you don't "deep cycle" it too often. A "dual purpose" marine battery is an ok compromise for single battery setups. The dual purpose battery will be more tolerant of deep cycles than a normal cranking battery and offer more cranking amps than a comparable deep cycle battery.
I have had a 900 watt rms amp in my boat for 3 seasons now and have never once drained my dual purpose group 31 battery even after playing the radio with the engine off for a couple of hours.
If you want to play it safe just get one of these jump packs:
You'll need a bigger one like that for a V8 so don't go get the $50 ones. The nice thing about them is that they can be used for your tow vehicle or to assist other boaters. The bad thing is that you have to remember to move it back into the boat if you take it out.