(1) Replace the dissicants with fresh ones.

(2) Only open the housing in a cool air conditioned room.  Don't open it
on the dive boat or dive site.

Andy

Following another answer, if you have to open the housing on the boat or
beach, leave it open long enough for everything to reach ambient temperature
before you close it again.

Jeanne

Something that has worked well for me is an air-blower nozzle for my BC
inflator hose.
Cost less than $25, and is smaller than a pack of cigarettes.

I use it to dry off the housing seals before opening them (make sure no
drops of water
get in housing).

Then when I'm ready to close the housing, I close it to within 1/4" of
sealing, and use the
blower nozzle to blow DRY air from my tank into the housing. Prevents
humidity build
up in cold water.

Many dive boats have blower nozzles on their dive tables, BUT... not all
blower nozzles
are being supplied with DRY tank air. Some boats connect blower to an
ordinary air supply
that is saturated with moisture. You need air from a SCUBA tank, or from
a tank filling system
that is supplied with dry, low humidity air.

Rick, to add to the good advice already given, try the following:

1. Assemble the camera and housing the NIGHT BEFORE with the FRESH
dessicant packs inside to allow more time to dry out the air both the
camera and the housing. (just rubber-band the housing shut, if you
want to avoid compressing the seals.)

By the way, you can obtain SPENT dessicant packs by the hundreds from
shoe departments (the managers will be happy to give you permission,
as they often are required to remove the packs themselves, in fact, my
local KMart manager set aside about 500 packs for me in 3 weeks- I
expressed my gratitude by giving her some scallops!).

You can REJUVENATE the packs, en masse, in low oven heat (about 220
degrees F. for 4 hours; watch the temp increases during re-heat cycles
to avoid kindling the packs!) Then store the rejuvenated packs sealed
in small resealable plastic bags, ten or twenty per bag, so that
re-opening a bag affects only a few packs (as opposed to repeatedly
re-opening a bag of a hundred, for example).

2. As was mentioned, if it becomes necessary to open the housing
between cold-water dives, first allowing it to warm up to ambient temp
helps to reduce subsequent housing moisture, but go one step further:
after reclosing the housing, place it in cold water (even ice water),
but do NOT immerse the lens. This will help keep any condensation
limited to the walls and not the lens. When you get into your next
dive, fogging will be less likely.

Of course, keep the housing out of the sun to avoid driving moisture
out of the camera and into the air in the housing. Moisture will be
driven out of the camera and into the housing by operational heat
also, so the practice of chilling everything EXCEPT the lens can be
useful, even if the housing hasn't been opened. During a dive, the
lens will cool down to the dew point much quicker than other housing
surfaces and it will condense a disproportionate amount of the
internal moisture. On occasion, there will be no condensation on the
lens during the first dive, then fogging appears on the subsequent
dives of similar temp, even if the housing hasn't been opened or left
in the sun, etc. I think the operational heat must have released
moisture to the air in the housing. Chilling as suggested can help
keep it off the lens.

I'll have to try the injection of dry scuba air when I've opened the
housing between dives. That makes good sense, too.

HTH.

Dave C  (please note: email address is defunct)

All the answers were interresting, but how about using defog on the lens
port?