In the beginning of the film despite going to bed without any clothes Daniel miraculously (and thankfully) gains a pair of briefs for the middle of the night mosquito swatting scene, before waking the next morning. Presumably this an effort by the director to keep the film rating as low as possible. But why not make him go to sleep with a pair of briefs on in the first place?
The goof item below may give away important plot points.
After both have been in the water for quite a while they drop their weight belts to make them more buoyant. At the end of the movie and after Daniel dies, Susan drops her tanks and submerges. That is impossible to do consider she is wearing some sort of suit which gives additional buoyancy. That is why they are wearing weight belts in the first place - to offset that additional flotation in their suits.