Boats don't need to be advanced! Why, you can just tie up several logs together and call it a day. This is the cheapest form of aquatic transportation, mostly used for it's ability to transport single units of any size. It offers no protection for it's cargo and, as it turns out, blowing up a few chunks of wood is not a particularly hard task.
Every faction posseses the skill required to construct this marvel of engineering, but using it is possible only in secure areas