Do check your O-ring for dirt, dust, crack and other abnormalities. Always grease your O-ring lightly before putting back to the groove and closing the housing. And ALWAYS check that O-ring is sitting nicely after closing. I am speaking from painful experience. :cry:
Also since you are first time using the housing, bring down t empty housing to test for leak (or lack of) before putting the camera inside. Empty housing is more bouyant, so you might want to tie some weight or put into your pocket (if the housing is small).
Read the instruction of your housing which should provide you with the info.