Hopefully it won't really need it. I've had about 3 instances of lens fogging in almost 400 dives with my camera, and never use desiccant.
Personally, I wish there was a smidge extra room in front on the right so I could put a Richard Franiec Grip on the S90 and leave it there, rather than taking it on and off (impractical) or simply going without (which is what I'll have to do).
I have had plenty of fogging, maybe it is the warm humid environments I usually dive in.
The grip, lol, forget that, not going to happen, no way. This housing fits the camera like a glove with virtually no extra room. Maybe you can put that grip on your spare S90, huh,

Actually the precision fit is what allows the ring controls to work, it is a good thing, not a bad thing. Don't need no stink'n grip.
N