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