Well the most important thing we need to know is the battery voltage. Drag out a voltmeter and check it, then start the rig again let it run a while or take it for a tide, shut it off again and take another reading, and post the results.
Could be a lot of things...
- Cold weather draining the battery.
- Short in the electrical system draining the battery
- Problem with negative battery terminal grounding to the frame
- problem with the starter
- problem with the alternator
- crappy battery
You can't even begin to diagnose a battery problem without a voltmeter....