Keep the interior of the housing dry. If any dust, dirt or sand gets in there, remove those. If there is any moisture in the interior, this will cause fogging when the housing heats up - from the camera or the sun. It sounds as though there is already water inside the housing, or water is getting in through one of the button or dials.
When you wash the sealed housing in fresh water, consider pushing and turning all the buttons and dials several times while submerged, this may dislodge any fine debris or sand that can become lodged. Every button and dial is a hole that can allow water, and only an o-ring or two are keeping water out. Also, remove the main o-rings and clean the channels.
After the camera is cleaned, leave the back open to dry. Later, seal up the housing without a camera and take it for another dive. You might put some tissue paper in the housing to see if that becomes wet or the housing fogs. If it does, there is likely a leak somewhere.
Good luck!
Craig