Just off the cuff thinking about it, you could probably (in World Machine), use a Meshify device on the water system to export it as an obj file, and place that in the Terragen world with an appropriate texture. Failing that, you could very easily create a mask for a general water texture in Terragen by using a "Select Wetness" device in World Machine (I usually add a Constant generator with the height at 0 piped into the precipitation map just to make sure it only outputs where water currently exists).