The troll of the thorn forest clearly knew how to build things, small houses, fences and tools, such as axes and fishing rods, they definitely held some level of skill in that department, so having them build a boat shouldn't be out of the realm of possibilities, unfortunately, communication was difficult, they actually seemed to understand Loimos more when he spoke in the death tongue than the pale tongue.
Those guys were awfully silent at all times, actually conveying things to one another by staring at each other and doing some eye movements, sounds meant nothing to them, so they actually got more of the undead's intent when he uttered words on the language of the dead, it carried more of his will, one might say.
