Before you can answer the question you'll need to know your daily amp consumption
. On a 26' boat with minimal draw it should be in the 50 AH range per day.
Add up everything you use per 24 hour period, i.e. - lights (10 AH), chart plotter (10 AH), Radio
(5 AH), etc. etc. etc.
Once you have your "Amp Budget" then choose batteries that provide double
or more in reserve capacity. Two six volt Trojan T-105s ($100 each) give you 225 AH capacity and that should do the trick on a small boat with no fridge or other electricity hogs. More AH usage???? More batteries.
Then figure out how to recharge them. Doubtful your Ooutboard alternator will do the trick. A KISS wind generator
($1,000) will give you about 45 Amps a day in steady (10-15 kt) winds. A Kyocera
135 Watt Solar
panel ($350) with Blue Sky Solar
Boost ($250) will do about the same in 5 hours of good sun.
A Honda EU2000 generator
($950) should charge two T-105's in an hour with a 30 Amp charger
. These gens are tiny, quiet and sip fuel
. Expensive, but a nice thing to have......
You may also want to buy an oil lamp
and some battery
powered nav lights for when the gasoline runs out, the sky clouds and the wind