The first problem stems from a mixed environment. Lenses which
were designed to focus in one environment (air) must now also be
able to focus through water.
The second problem is that water
absorbs light much readily than air. As a consequence, there is
less light below the water than there is above --the photographer
can either expose for a subject above the water, or below, but not
both. So how does the photographer get around these limitations?
In the first case, the best way is by utilizing a split-diopter
accessory lens. A split-diopter lens is merely a screw-on filter
that fits over the primary lens. Split diopters are essentially
photo-quality magnifying lenses cut in half and mounted in a
rotating bezel. (most camera retailers either carry or can order a
split diopter from a distributor).