Thanks a lot for the feedback!
For barrels above 12, other than keeping them on the map you can spend them with the Heavy Weapons berry ability to deal extra hits at the ship or spend them in the daylight to battle using the hireling (within this battle you can spend them to ignore hits you take). The Quarter action also takes barrels from the ship, so unless you have extras, recruiting away from the ship costs you more actions to do. I do believe the faction could work with 18 barrels instead though, I didnt really do optimisation on the number.
The canals also make the clearings rivers for everyone, so while it admittedly doesn’t have the greatest effect on most factions, factions with Swimmers can ignore rule while moving on them, riversteads counts them, and riverfolk can recruit on them. The main goal with them was to make a clear AOE for the ship that the enemies can control by destroying the connection to the main river or destroying a new canal so you can’t move there in birdsong for the battle with hit protection