Eau you're right. There's loads of water.
The Channel - crossed and recrossed by many of the characters: Tony, Emily, Ollie, Mark, James, Vincent, Baptiste and his wife.
When Tony confronts Vincent, Vincent is standing in the lake fishing.
The police drag the lake after Ollie goes missing.
Vincent frequents the pool and lake area.
The dream sequences with Ian and Mary on the Indian Ocean.
Tony's sunlit and happy swim with Ollie being contrasted with his desperate dark swim to shore after sinking Ian's body.
There is also a theme of parenthood. Almost all the adult characters (Tony, Emily, Mark, Ian, Mary, Baptiste, Ziane, Laurence) are either shown interacting with their children, have photos of them prominently displayed or refer to them. I think that Malik is also a father.