All batteries that I am aware of lose their charge over time if they are not maintained on a charger, and some more than others. Lead acid, for example, looses its charge faster than AGM's.
Depending how your boat is wired anything that remains "on" when the ignition is off and the battery switches are off will slowly deplete the battery(s). Co detectors, MerCathode, clocks, stereo memory, anything that goes into "standby" mode when it is switched off. Bilge pumps - yes and no, because when they are not running they do not consume any power. An exception might be those equipped with electronic "floats" or electronic high water alarms - which may require a constant 12V feed for their sensors.
You should be able to figure out what the total draw of the consumers, then divide that into your batteries amp hours to get an idea how long it should last.