A second adds too much weight so we've got a single Optima blue top AGM. The outboard will charge that along with a solar charger. The only electric draw will be the GPS and tiller pilot on the rare occasion we decide to use it. The lights are all LED and a negligible power drain.
The kicker that allows me to break the backup battery rule? The 9.9 Honda 4 stroke outboard has a pull start.