Sorry, about the delay... I just caught up with this.

Besides the refraction issue already mentioned below there is another property at work: pressure.

When swimming, the water pressure presses against the lens of the eye and permits focusing easier. As we age our muscles which are used to squeeze our eyeballs and subsequently flex our lenses get lazy and weaker. As a result our focusing ability decreases.

When young, with 20/20 vision one can usually clearly focus text within 6 inches of ones eyeball. As one ages, the distance gets further, inch by inch, as the muscles can't flex the lens as well. (It's actually a combination of the muscles and the lens' increasing rigidity) Added pressure can aid the muscles natural pulling and pushing of the eyeball lens to focus and also make it temporarily more flexible.

In the not so distant future (say 100+ years from now), I suspect for people with severe ocular issues, small mechanisms will be developed which are linked to identified/mapped nerve signal impulses and will aid the muscles flexing of the lens. Optical glasses will be augmented/replaced by new servo-motored assistants. These folks will recover almost perfect, albeit costly, vision.