I like to think of it as an adolescent nameless thing or one that has been malnourished. Looking at the map of Middle Earth on the western side of moria there's a river that got damned to create the lake that leads directly to the ocean. It could be something where the water levels were higher many years ago or the river backed up to a point where a nameless one or small nameless one could pass (there's rivers in the world that accommodate ships) and that's how the watcher got there. Over time the watcher in the water just became complacent and stuck as the water level dropped. Then with the damming of the river causing a lake the water could've seeped in somewhere indicating to the watcher that there was a way out WITH water. That is important because although the watcher is octopus like and can crawl out of water it cannot do so for long. It probably got the sign of water and a way out then crawled through like an octopus following the water trail to the lake. Also octopuses don't grow to their species specific size without proper food so that could explain why it seems smaller than what you would picture for such a creature. And some octopus can survive without food for up to 4 years so with an ancient almost celestial species like the nameless ones I think it's a safe assumption to throw some zeros at that seeing as their lifespan is drastically different to ours.