O que foi o “Observador na água”?

Parece ser um tipo de lago subterrâneo vagamente definido Kraken ou um espreitador de masmorras semelhante.

Fora da A Irmandade do Anel é mencionada apenas em A História do Senhor dos Anéis , que contém algumas das notas de Tolkien.

De Watcher in the Water na Wikipedia:

Since Tolkien never explicitly stated what the creature is, others have felt free to speculate on its identity and origins. In A Tolkien Bestiary, David Day calls the Watcher a kraken; however, he also implies that there are some differences between the kraken of Scandinavian folklore and the Watcher in the Water. However, Tolkien never called the Watcher a kraken nor described the presence of krakens in Middle-earth. In The Complete Tolkien Companion, J. E. A. Tyler postulates the Watcher was a cold-drake: "...these dragons rely on their strength and speed alone (the creature that attacked the Ring-bearer near the Lake of Moria may have been one of these)." Another writer compared it to squids.

A resposta aceita é excelente, mas gostaria de acrescentar que simplesmente não sabemos.

Nas obras, não fica nem 100% claro que o Vigia é uma criatura ou um número de criaturas independentes semelhantes a cobras trabalhando juntas.

‘I felt that something horrible was near from the moment that my foot first touched the water,’ said Frodo. ‘What was the thing, or were there many of them?’ ‘I do not know,’ answered Gandalf; ‘but the arms were all guided by one purpose. Something has crept, or has been driven out of dark waters under the mountains. There are older and fouler things than Orcs in the deep places of the world.’ He did not speak aloud his thought that whatever it was that dwelt in the lake, it had seized on Frodo first among all the Company.

Embora seja extremamente provável que fosse uma criatura, e a teoria de Kraken se encaixa, não podemos dizer com certeza sem um grau de especulação.