How big is his boat? Can you give me some pictures?
To use aluminium as cabinet, make sure you use only aluminium, or only aluminium can expose to water or it will create galvanic corrosion. Normal water contain many ions so it is a electrolyte.
If your water is salt water, aluminium can't be use for a long time. Cupronickel will be much much better in this case but they are more expensive than aluminium and hard to brazing together. Plastic are other choice but plastic can't withstand in scorching for a long time.
For cheap and long durable, lead-acid battery is the best if weight isn't a problem. Sealed acid battery are better for normal use but less reliable in hard conditions. To use lead-acid battery safely, you should take it away from the boat to charge to prevent acid leakage. Like spec have said, the metal battery cabinet should be protected by acid-resistance paint or epoxy.
Use Li-ion or Ni-MH battert will be better than acid type: Smaller, lighter, higher energy capability.... but will costly.
You have said that the stereo use 110V so it certainly need inverter. You may consider to use C-L-C filter to smooth all ripple cause by inverter or the stereo will sound like a swarm
. Have you ever think about disassemble the stereo, and supply it directly to the board without through the it supply circuit.