(Disclaimer: I've not seen that documentary so I'm sure what exactly it said.)
In a literal sense, Angkor Wat was built upon a sea of groundwater. The city was built in a very wet and water-rich area; much of this water found its way underground. At the lower levels, the water fills up all the pores and holes in the sandy soil. The water table helps firm up the upper levels of soil, upon which Angkor Wat's foundations sit.
In recent years, the regional water table has been lowered through rampant pumping of underground water. It is feared this would literally undermine the ancient city's structural stability.
Figuratively speaking, Angkor Wat thrived on its water resources. It provided a massive irrigation system fuelled by a network of water reservoirs. This enabled the high agricultural productivity that allowed Angkor Wat to maintain a large population. It was the failure to maintain these water distribution systems that eventually led to the city's abandonment.