I am planning to purchase a portable 12v charger/power supply for the boat
rather than installing a second battery. My 12v power draw is negligible
(engine starter, cd player, depth sounder) when not running.

This is one unit I am looking at:
http://www.break.com/movies/houselights3.html

Any recommendations or am I being foolish by not installing a second battery
with A/B switch for my modest 20 foot runabout?