Help!! Battery help.

In my 2019.Malibu 25 LSV (boat), we have 2 Minimaxx 3k’s from D4s and the 2 Malibu Wetsounds amps, those adding up to about 2000 watts all together. We have an 8k watt system in the boat but we have shitty power. We have 3 oreillys 102ah agm batteries that are not doing it. 2 for the audio and 1 to start the boat. We also have terrible voltage. It drops to like 11 volts since we are not always using the alternator because we’ll park the boat and go swimming for hours while also bumping music while just off raw battery power. We were looking into lithium but we don’t know much about lithium so we need some help. Please give us some answers because we want to be out on the lake bumping. Try to give us some cheap options, under like 800 bucks but we can change that if we need to. Also feel free to ask for any other details about the system.