Yes, the Cold Cranking Amp, CCA, specification is one way of comparing batteries, but might not be the best parameter to use when comparing marine batteries.
The CCA specification defines how many amperes may be continuously delivered for 30 seconds, at 0°F, while maintaining 1.2V/cell, or 7.2V across the terminals of a 12V battery.
The battery engineers balance the trade offs between short-term bursts of high current and long-term sustained delivery of low current.
A starter turning over a gasoline engine will draw about 50 to 200 amps. A starter turning over a diesel engine can draw up to 1,000 amps. For a gasoline boat, any battery in good condition will provide sufficient current for starting.
The OP has a 2004 220 Sundeck. It has one battery, with a second battery optional. The battery switch is also optional. The OP says "batteries", so there are two.
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It is true that any / every new marine battery will start the OP's engine and run the stereo while on-hook. The OP can select fresh batteries based on cost and the boat will work.
If the OP wants to maximize the number of years between battery replacement, then go w/ AGM over flooded cell. If the OP wants maximum run time, then select the Lifeline GPL-31XT.
Regardless, the batteries should be the same manufacturer, date code and model.