Cara Delevingne & Selena Gomez -- Getting Wet Together

It's Selena Gomez's birthday, but we're getting the gifts -- behold Selena and Cara Delevingne showering ... together ... in bikinis ... on a yacht.

Our favorite new BFFs made the sexiest pitch for water conservation while continuing SG's bday week celebration in St. Tropez -- but this time they had company in the form of Tommaso "Tommy" Chiabra.

In addition to ruining an otherwise perfect moment ... Chiabra has been seen with Gomez a lot recently, and has also been linked to 'Gossip Girl' Jessica Szohr.

Hate him. Love the pics.