Batteries begin losing capacity right from new, and the process continues until they are deemed no longer useful by individual owners.
Think about the start battery in your car. It starts the car every morning for years.....until it doesn't. And you wonder why it started yesterday but won't start today.
Because it takes less than 1AH capacity to start the average cruising boat diesel
. That's right, less than 1AH.
Yes, it will draw 150amps or so, but only for a few seconds until the engine starts. Multiply it out, and you'll find that the total energy used (amp-hours) is less than one AH.
Now, while you contemplate all the math and things which can intervene in reducing battery capacity and useful life over a multi-year period, order a new start battery.
A plain old group 27 or group 31 will be fine, and will last as long or longer than the Optima. EchoCharges love 'em :-)
Also, keep in mind that a battery with almost all it's capacity eroded may still indicate normal voltages. Voltage does NOT equal capacity
....only state-of-charge (SOC). A bad battery may have a 100%SOC, but only a small percentage of its original capacity remains -- e.g., if the plates are heavily sulfated and only a fraction of their original area is available to store and deliver electricity.