George Clooney and Brad Pitt at the charity dinner for Not On Our Watch
Pascal Le Segretain/Getty
08/27/2008 at 11:00 AM EDT
and Brad Pitt
lived la dolce vita
in Venice Tuesday night, partying into the wee hours.
In town to promote their film Burn After Reading
, which opens the film festival Wednesday, the pair first attended a black tie fundraiser for their Not On Our Watch charity. Some 200 Italian VIPs – plus their costar Tilda Swinton – attended the dinner at the Hotel Cipriani on Giudecca Island.
"They both have such passion and such dedication," a guest told PEOPLE of the pair.
After the event wound down, the two actors partied on with some 20 fundraiser guests, arriving at the Cipriani's poolside bar just after midnight.
"We've had a lot to drink," said a jacketless Pitt, 44, laughing and raising his champagne glass. A tan and trim-looking Clooney, 44, had his shirt unbuttoned.
After chatting with some guests – including Clooney's pal, British TV personality Mariella Frostrup – the actors held court on a wicker sofa by the pool. By 2 a.m., a pair of women decided to strip down and take a dip as Clooney looked on from the edge.
Later, the actor shimmied while the hotel's guitar player strummed Gloria Estefan's "Mi Tierra."
"In case you didn't know, the water's salty," Clooney yelled as he headed upstairs at 2:15 a.m.
Pitt, who arrived in Venice Tuesday with sons Maddox, 7, and Pax, 4, stayed chatting until just before 3 a.m.
Get your fix of Brad Pitt! See all of his PEOPLE covers!
See George Clooney's first PEOPLE cover