Welcome to the forum.
The best option is to maintain the systems, including the batteries so all are okay.
The best configuration on a boat is to have redundant options for starting. That includes multiple isolated battery banks, plus my boat has a third bank just to start the generator, that will then power the on board battery charger.
With this configuration I would have to try really hard to attain a flat batteries / no start condition.
A well maintained flooded cell battery typically lasts about 4-5 years. An AGM battery lasts longer.
My selection is the Lifeline GPL-31XT because the capacity is 125Ah, 25% more than other brands.
https://lifelinebatteries.com/products/marine-batteries/gpl-31xt/
A battery is considered End of Life when the measured capacity is less than 50% of the nameplate rating. My Old School method is to do a run time test, to quantify the run time, compared to the 20 hour spec. I use my carbon pile load and my DC ammeter and my DC voltage meter for the test.