If that's all you plan on running on your boat you answered your own question. Two $100 MARINE batteries linked together through a battery selecter switch will last you three or four years if you maintain them during the winter. I ran the same setup on my boat. The engine, fish graph, autopilot, radar, temp probe, fm radio, down riggers, washdown pump and interior lights were on the main battey. Safety stuff (VHF radio & gps/chart plotter) were hooked up to the spare (isolated) battery. YMMV...
