Good place to start.......
12 Volt Side of LifeDeka Marine Master...........Deep Cycle/Starting.....that means it is a so/so battery. Not really a Deep Cycle, not really a good Starting Battery.
Can't tell amp hours/group.....that info is probably on side of battery case
If it is a group 24/65amp hr, group 27/80amp hr, group 31/100amp hr
All pretty low.
2 batteries are good...should be same type/size/age
Add 'distilled water'.......flat caps pop off use flat blade screw driver tip in notch center of flat cover
Disconnect......remove NEGATIVE cable first then POSITIVE.
Connecting......Install POSITIVE cable first then NEGATIVE
NOTE: your Black cable is POSITIVE
I would drill couple drain holes in battery box.
ON EDIT: Swapped Positive/Negative.....THANKS! :B