When we recommend battery setups, we use max wattage as the main determining factor. Obviously, you will almost never have the stereo at the maximum level for any extended period of time, but estimating capacity this way will give a bit of a safety net.
You will have an estimated 1800 watts on tap. This converts to roughly 150 amps of current needed to run your stereo at full volume.
Now you are thinking, "Well, two 115 ah batteries should work perfectly and give me a little less than two hours of playing time". This is an easy assumption to make, but unfortunately it is incorrect.
The standard for ah rating is a constant draw over a period of 20 hours. Meaning that, for a 100 ah battery, it can discharge 5 amps for 20 hours without failing - or at least this is how the rating comes about.
Since capacity decreases at an increasing rate as amp draw increases, this measure does not mean that a 100 ah battery will provide 100 amps of current for one hour. The reality is much more like 40-60 amps/hr - and this is assuming the batteries don't typically fall below 20% capacity (Recharging from below 20% repeatedly will significantly reduce the life of the battery).
Basically, for your wattage, we would recommend 3-4 Group 31 120 ah batteries. This would give you plenty of power for your stereo, and several hours of playing time without depleting the batteries below 20%.
As stated before, the charging system in your boat is very small. I'm assuming you will be keeping your stereo battery bank separate from your boat's charging system and you will be shore-charging this bank when not in use. You should be able to charge 3-4 group 31 batteries with one charger without any troubles.
I realize that is quite a bit to think about, but let me know if you have any additional questions