Since you wrote "INEXPENSIVE" in your original post, don't buy an AGM battery. They are awesome, but the benefits they offer will likely be irrelevant for your application thus they are not worth the extra cost IMO. AGMSs are great for situations where the battery cycles through partial discharge and partial recharge cycles most of the time. I cannot fathom that happening in your application. You will very likely have plenty of time to recharge. You should just get a regular flooded lead acid battery and keep it on a charger that has a float phase every time it's not being used.
AGMs are popular for people that spend a lot of time anchored and not connected to shore power. In this use, they are often discharged to 50% state of charge and re-charged to around 80% (end of the bulk charge phase) state of charge. To get them above 80% SOC, you need to go through the absorption phase which takes a really long time and most people that anchor out multiple days in a row don't want to run a generator that long. Leaving them at 80% SOC does not hurt an AGM like it does a flooded lead acid battery.
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