"Can't it just sail south, down Niben Bay and to the sea?"
Nope. Leyawin is in the way
"How about to Anvil?"
Most of that route is landlocked, and there's no way for the ship to get to Anvil. So how the heck the ships in the waterfront head out to sea? Is there a group of strong-armed men near Leyawin who literally pick up the ships and carry them the few hundred feet to the sea?