Beckinsale took to Instagram on Monday to share a sexy mirror selfie of herself rocking a floral two-piece, while 18-year-old Lily, also sporting floral bikini bottoms, boasted about her "really busy day."
Beckinsale split from Lily's father, Michael Sheen, in 2003, but the actors have since remained very friendly -- and not just for the sake of their daughter.
During an interview with ET in December, the 43-year-old actress revealed that she "just really likes" Sheen.
"I think we both really felt like, we really agreed on what would be the right thing for Lily. And I think, you know, not hating each other and fighting and all that would be awful for her," she added. "I think we've also got a really similar sense of humor, and I do find that humor can save an awful lot of things and help a lot of things."