Based on the amp/hour ratings of known and conservative group 31s, some may be over estimating the A/H rating of the Interstate 29. But in any case, it is a good battery with tons of capacity.
With Class D amplifiers your current usage will be less.
You do not want to regularly run these batteries down below 12.0 volts. So if you discover that two group 29s for example, are not sufficient, add a third immediately before there is much age on that bank.
I would not isolate the stereo bank from my boat's charging system. An infinite number of fully charged batteries do not represent a challenge to your alternator. By allowing the alternator to service the bank throught the day, and until you have a stay at rest, you are maintaining a higher voltage and greater reserves. Then if the bank is deeply discharged after a long stay at rest, you have the option of not re-introducing it to the charging system.
Your amplifiers are playing AC based music content, unlike the constant draw of a DC bulb, so the current draw will not approach simple wattage/voltage/efficiency calculations. The stereo with a charged battery bank is not going to be too much load for the alternator.
Your amplifiers are going to drop 25% of their 14.4V power rating with a more realistic 12.5V supply. And they will lose another large % of power when the voltage drops to 12.0 volts or below. So keeping that voltage high is important to the audio system power and important to the longevity of the batteries. You will get an inordinate increase in number of cycles if you avoid deeper discharges and an inordinate decrease in number of cycles with deeper discharges. It's not a linear ratio.