a 450watt amp shouldnt really be enough to drain your batt. in any less than 4 hours assuming that its 100% at the start. A 2nd batt. is REALLY nice to have though too. and also, if you dont want to manually switch between the batteries everytime you float with just the stereo they do make a thing that will run on both batts. to charge them while the engine is running and it will automatically switch to one batt. when the engine is turned off.
I don't do much floating because the seadoos like to spin cirlces around me and flood my boat.