The lawn tractor battery will work, as long as the charger is not running at the same time. If you just want to listen, a ground (pound in the ground house rod), and a long wire antenna
will work fine. Anything else will require a tuned antenna
or a tuner and long wire antenna or something like that.
I do believe that you would be operating outside of the law if you transmitted, without a ground station licence for the marine
unit, to operate on marine
bands. Listening is of course fine.
If you want more info, try finding some local Ham operators, or look into becoming one yourself. You can also power the unit with a DC power supply designed for HAM radios. Most likely you would need something of around 25 amps, for transmitting, much less for just listening.
If you already own a charger (even a cheap
DC car charger) an old battery that won't start anything anymore will work for some amount of minutes before needing another charge. I have used "Dead" batteries in this way before to power a ham radio