Upgrades are always fun. So exciting. I'll share my upgrade path. Maybe it'll help the community.
It came with a nice AGM replacement from the previous owner. I left it alone and use it as a starter battery. In addition to that, I added a Lifepo4 30ah pack under the seat tray. Lithium iron phosphate chemistry is known to be stable and safe. I would avoid any lithium polymer packs.
I was going to make a bracket, but just decided to hold the battery with heavy duty plastic ties since it's so light. Most packs will come with a nice BMS, so it'll prevent overcharging, cut off at low voltage to prevent cell damage, and does a decent job balancing the cells. The only thing you'll need to keep an eye on is low temperature operation. Normally, we'll be out of the water when temps are in the 40's F, so this won't be an issue. The power density and value is much higher than your conventional battery. I only paid ~$100 for the 30 ah lithium pack.
There's several ways to connect this.
1) Add an XT90 connector and manually connect it to the main battery when you put the craft in the water. You need to disconnect it after. It is not recommended to leave it connected to the AGM battery. It will always try to hold the AGM battery at a higher potential and self drain.
2) Add a high current relay and have it energized with the accessory power. This is similar to 1 and will automatically disconnect when accessory is off.
3) there's ways to isolate it to run devices, but that just adds electrical complications and more items to fail.