Like Bruce, I ALWAYS carry a quick jump battery box and cables on every trip!I have the cables and to Marine Maxx trolling motor batteries. I would think with a barely dead Optima jumped to one of those Marine Maxx's I'd be fine. Those battery boxes cost about as much as another battery. I may end up just getting a 2nd battery in the back for electronics and stuff. If I do that so my starter only is for starting, can I assume my 2003 Mercury 125 will have enough generator to always charge my starter? I only have a 3 bank charger and with 4 batteries either the starter stays out or maybe I can run starter and electronics in parallel?
I have not charged my starting battery since I put in boat. I am only running a 9.8 hp so I don't know if that has anything to do with it. But I'm pretty sure you won't have any issues unless u gots a short some where drawing power. I run/charge my trolling batteries paralleled up. I have gone out for 12 hrs night fishing running my trolling motor, nav lights, fish finder, live well, drop lights, KC off road lights, led interior lights and can run all night on two fully charged blue tops. I do run a ever start for my stater battery. If you carry some simple tools and you are able to access your batteries fairly easy you can always swap a battery around so your able to get off water. I have had to do this before.
I did have some issues in the past with losing my charge in the middle of night but I changed some things to improve my battery life. Live well timers are a good thing to have. I have been trying to change all lights out to LEDs. I also turn my fish finder back light down has well. I just try to remember that the more I conserve the longer I can stay.