When Dan is first setting the charges at the bottom of the ocean, he is wearing swim fins. After he starts experiencing "rapture of the deep" symptoms, he's kicked them off and you can see the swim fins on the ocean floor. As a disoriented Dan starts floating toward the surface, he is barefoot. After Flipper's rescue , as Dan starts swimming toward his boat, he's wearing swim fins again, but as Porter and Bud spot him and pull him into their boat, he's barefoot again.