When the monster snaps Fred's lasso and he and Velma fall into the sea, Daphne comes to them on her jet ski and she is shown to be in full scuba gear. But when the shot changes and the camera switches to above, all of her scuba gear is gone; including her flippers. Furthermore, the rope, rather than falling as well, hangs in mid-air for a second and then disappears.
Velma is able to locate things near her without her glasses before diving, despite not being able to tell the difference between Scooby and Motoshondu underwater after her goggles were knocked off by the sea turtle.
Velma is given her goggles back by the monster, after a sea turtle knocked them off. How can she put her goggles back on underwater? There would just be water in them and would be the same as opening her eyes underwater.