You can get by with one battery, but you are going to have to put up with EMI (Electro-magnetic interference)noise on your fish finder at the same time. There are noise suppression diodes available that cancel that out and allow the use of one batt. Using a PWM motor control allows you to regulate your bait tank pump into an energy efficient machine(vs. reverse polarity or running on 6volts) and you could get by all day on the water with a simple and inexpensive 12v 8amp hr. batt. If your plan on using lights bump up in amps(12amp) and stick to LED lighting. In regards to batt. chargers, I also got rid of the cheap cabela's model and bought a unit from an outfit called Deltran. They have a 12 volt, 1.25amp fully regulated charger for under $60. It takes good care of my batteries. Thats my 2 cents.
Link is
www.batterytender.com
Good luck