After a while you might find that the buttons on your underwater camera housing are stiff or some of the buttons work fine on the surface but not at all underwater.  This guide shows how to remove and lubricate the buttons on your housing which in most cases will fix the problem.

Before trying anything else you should follow the manufacturers recommendations for cleaning your housing, the directions for housings made by Canon can be found here. 

Here is a list of the tools you will need to remove the housing buttons:

Small flat head screwdriver Small pair of pliers Pick with a dull point O-ring lube

Start by removing the e-clip holding the button mechanism together.  The clip can be removed by using a small flat head screwdriver and prying until the clip comes off. The e-clips have a tendency to fly off, to help prevent the clip from being lost you can place the housing in a clear plastic bag and poke a hole for the screwdriver to fit through. Note: Some buttons may have a rubber tip on them like the ones shown to the right, the tip can be removed by pulling on it.
Now remove the button arm that was held in by the clip.
Not all buttons will have 2 e-clips but this one does, removal of the second is the same as the first one.
Now the button can be easily removed, clean the button shaft with a damp paper towel and inspect it for any corrosion or damage.  If the button shaft shows any signs of damage the entire housing should be returned to the manufacturer for repair.
Carefully remove the o-ring, here a brass pick that has a dull tip is used to prevent the o-ring from being damaged.
Inspect and clean the o-ring with a damp cloth, as you can see this o-ring was pretty dirty. Use a damp clean Q Tip to clean the o-ring seat, after cleaning make sure no fibers or other debris is left that could potentially cause a flood.
Place a small amount of silicone lubricant on your finger and spread it around.
Lubricate the o-ring by rolling it between your fingers.  Once the o-ring is lubricated place it back in the button shaft, be sure that the o-ring is fully seated.
Installing the e-clip can be a challenge, the easiest method I have found is to use a small pair of pliers, place one jaw on the e-clip, the other on the button shaft and gently squeeze until the clip pops on.
After assembly it’s a good idea to take the case on a test dive without the camera to be sure there are no leaks.