What about an underground cave system thats big but doesn't currently go all the way through from Roe Lake to the main as yet unnamed lake. You could sail small boats in so far and then have an underground set of locks with maybe a cavernous route from various places near to the Roeborder Ridge Fort down to it. That would make for an interesting reason to go subterranean. There would be no end of adventures protecting ships and dredging up loot from lost ones at the bottom of underground cave systems. A real magnet for intelligent monsters or pirates planning ways to scuttle boats.