Some clarifications are in order here....
1) batteries come in three types ... Cranking... deep cycle and dual purpose. ( within each type ther are groups that determine size group24 being averagfe, group 27 the big brother)
Cranking batteries are made for short bursts of power, their plates are thinner and designed for sudden short duration heavy loads..they do not withstand many deep discahrges before their plates become sulfated and ineffective.
deep cycle batteris are the work horse of the battery clan, they are designed for long term discharge/ recharge cycles, but because their plates are thicker, they sulfate and can be ruined when asked to crank high horse power engines on a regular basis..
dual puropse batteries are perfect for fisherman, they sport alternating thin and thick plates, they can with stand more deep discharges than a cranking battery and can serve as a cranking batter rather reliabily, but lack the sustaind cranking ability of a true cranking battery...
Chemistry: Marine batteries come in three chemistries, flooded, AGM ( absorbed glass mat) and Gel...
Flooded batteries are the least expensive and most common, they can spill acid and must be maintaned to be effective..
AGM batteries are completly sealed, they charge %40 faster than flooded batteries and have a lower self discharge rate.. they can be mounted in any posistion except upside down.. they are dual purpose batteries and IMHO the best bang for the buck..( I run two of these in my 18' skiff)
Gel.. very simular to AGM, except are more of a deep cycle battery althohg made to be dual purpose..
Switching...the old Bat. 1, Bat 2 or both switches are obsolete.they leave too much to chance, to keep both batteries charged, you must manualy switch between batteris... the other problem is in battery isolation, this system does not allow for starting isolation , and often times voltage spikes can damage electronics, at the very least interfere with it's operation..
a better sytem is to use a series of switches or better yet a switch that isolates the system and each battery when starting and combines them when charging via a voatage sesitive relay, that combines both batteries during charging and isolates them during discharge or starting, the price of the relay and switch is modest and make for an extremly reliable system that gaurentees that both batteries remain charged but delicate elctronics are protected when you start you engine(s)