When Sabrina has to get to the boat to save her mer friends she jumps in the water wearing flip-flops, and a dolphin takes her out to the boat. After the crisis is over and she is back on the dock she is still wearing the sandals! There is no way they could have stayed on during that wild ride.
OK, I haven't seen the episode, or movie, or whatever, but I'm assuming this Sabrina is THE Sabrina; the Teenage Witch of TV fame. Am I right? OK people stick with me here. I'm going to assume that you are right, that there is no way in the whole wide world that Sabrina's flip-flops/sandals could possibly have stayed on during her "wild ride" with the dolphin. I'm not even going to address the idea that they might just have stayed on, and that you can't possibly prove that in the best of all possible worlds they would never would. Oh no, that path leads to madness.
Let me just ask this: If Sabrina is a witch and she has magical powers, what makes you think she couldn't use her magic to preserve the integrity of her flip-flops? Magic sandals? Can you say pre-teen plot device? Hey, from what I know of that show, stranger things have have made it to the air.