I keep the lens on the camera. If you put it in a pocket it will be ruined, count on it. Some people will say that is what they do, nonetheless, it will get badly scratched.
I have two different lens systems, one is threaded and the other is bayonet mount. I have the Inon AD bayonet lens holders on my strobe arms to carry one wide angle (Inon UWL165AD) and one macro (UCL165AD). If I am using my threaded wide angle then I just shoot wide angle. Threaded lenses are just too much hassle to R&R underwater.
Most of the time I do not change lenses underwater even with the AD lenses because too much is happening, the group is moving down the reef and just cannot tarry to try different lenses on each subject. I try to plan what I might shoot before the dive and then load that lens and may or may not change during the dive, probably not, unless something really cool shows up in my view finder that warrants a change.
Sometimes I forget that no lens is a good thing and may start out that way and then install the lens, wide or macro, once on the site and see what opportunities are available, this for places I am not familiar with in advance.
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