Who were the Watchers?

The Bible says the Watchers are Angels, "sons of God", but this isn't explained. Since they could have children with women they are biologically humans. This is the basic definition of what a species is; if a pair of animals can have fertile offspring they are the same species. This is more diverse in humans than people used to believe. Genetic studies have shown that humans interbred with archaic types previously classed as other species, such as Neanderthals, denisovans, and some others too.
Enoch says the Angels looked like "white men" (87.2) and so did Noah (106.2), it seems.
Another question is what were the Watchers watching? You might assume they were doing some astronomy since the ancients do seem to have been very interested in observing events in the sky. On the other hand, if a group of astronomy students dropped out from an ancient observatory to get married, you wouldn't expect them to be remembered in the Bible as fallen Angels and the sons of God. I do have a theory, which is circumstantial and not provable, but perhaps one day the Watchers will appear and explain themselves.
Recent astronomical research has revealed that most of the stars in our galaxy formed between 12 - 9 billion years ago whereas our Sun is relatively young at only 4.5 billion years.

The Age Distribution of Potential Intelligent Life in the Milky Way

the majority of intelligent life in our Galaxy is likely to be billions of years more advanced than we are, implying
that we are one of the younger civilisations in the Milky Way.

it is likely that ETC [extra terrestrial civilisations] would be immensely different from our own civilisation and their
technologies would probably appear like magic to us. ...[page 27]

My theory is that, at some time in the distant past, due to some circumstances, we came under the protection of one of these older civilisations, and because they weren't very local to us, they trained some humans to act as watchmen and they are The Watchers. These watchmen would need a base, perhaps in space, special training and equipment, and some genetic modifications - to provide them with a decently long life-span of maybe a thousand years or more.