Bateaux were shallow draft boats intended to not only sail shallow waters, but be transportable over land.
Portages were those areas between bodies of water or portions of rivers not suitable to navigation, where bateaux were moved overland.
Forts, such as Forts Edward, Ticonderoga, St John, Niagra, and Stanwix were paced to guard these portage areas.
This is not represented in BoA. The Mowhawk river cannot be navigated, nor the Hudson, north of Albany.
The whole historic reason for using that lake/river complex for movement/supply is not operative in the game.
Is this being addressed in BoA 2?
Two sources:
Richard M. Ketchum, Saratoga, Turning Point of the American Revolution. Starts out with an account of Benjamin Franklin and his Congressional delegation's journey from Manhatten to Montreal in the spring of 1776.
http://www.dmna.state.ny.us/historic/articles/bateau.htm A good net article about the topic.