Director Riccardo Paoletti lost seven kilos during the shooting due too the difficulties of making ends meet with a really low budget.
The underwater scenes were shot in a swimming childbirth pool during a stormy day. All the murky effects were added in post production.
The film's subject lake, located near the Village of Stia (Tuscany) Italy, has some historical basis: "Located south of Mount Falterona, the small Lake of the Idols is the most interesting archaeological site in the Casentino. Very important artefacts from the cult of the Etruscans have been found here including many votive offerings, hence the site being called the "Lake of the Idols". Hundreds of bronzes have also been recovered, some of which are now preserved in the British Museum in London and the Louvre in Paris." - Source: italythisway.com.
Some scenes were supposed to be shot in sunlight but since the forecast was adverse the director choose to take advantage of the gloomy atmosphere and shot in the mist.
The actual Lake is really in Rieti.