Very cool project, please keep us updated!
Are you using standard lead-acid batteries, or going more high tech?
On marine applications, 6v batteries have several edges over 12v batteries, because each lead-acid cell produces 2 volts and in a 6v battery, you have much larger individual cells because the battery has 3 cells, not 6. These larger cells are more durable and have higher capacity. They're also easier to isolate should a cell fail.