I have the same problem and haven't found a solution yet!
There was a discussion about this on the scubaboard
here. There doesn't seem to be a definitive answer, some sugguest using this rain-x procuct, I think its something like Deep thinker mentionned but I'm affraid of putting any product on the housing. Mask antifog was also suggested.
The most reasonable suggestion would probably be to give it a shot of air from an octopus. Since that isn't possible while freediving maybe blowing real hard on the lens would work? I'll give it a shot this weekend.
