"Honey, can you put a little sunscreen on my back?" / "Sure. What do you want? The SPF H20?" / "What's that?" SPLASH! This is a harmless, fun prank, unless she's hiding her cell phone some place very creative. I'm not going to say specifically where, because I'm not that creative. It looks like all will end well with this prank, as long as this isn't the hundredth prank in a row he's done to her. There are entire Youtube channels dedicated to a boyfriend pranking his girlfriend. You better be sharing that ad revenue, dude. No one is watching for you. They're watching for her!
I used to prank my mom whenever she came home for a nap. Looking back, that was incredibly cruel. She was just trying to take a nap in between working double-shifts.