thanks for all the replies. I am now convinced that this was causing the fogging. Interestingly it never happened before with same procedures when shooting stills only. but with video operation and thus warming up the air inside the aluminum housing it obviously triggered the fogging. As a matter of fact on the last dive in Dirty rock were 2 whale sharks turning on and on for about 30 minutes and my lens was completely fogged....I almost bite the mouthpiece of the regulator.....Lesson learned!
I was there the last 2 weeks (mid August till end of August this year). we did not really notice a real El Nino effect. Water temperature was about 26 degrees of Celcius with some thermoclines of down to 20-21 degrees. Schools of hammers were there (more than in my last visit 2 years ago), sometimes with more than probably 100-200 animals, but I had the impressions that they were not so active on the cleaning stations anymore (and this situation is usually the only occasion of close encounters).
Seen also whale sharks (about 5 within the 2 weeks), galapagos sharks (many), silkies and silver tips, blacktips and off course white tips all over. Also 2-3 times tiger sharks (but not that close). And they were not anymore in Manuelita channel. Top diving places for the hammers were Dirty Rock and partly Alcyone (in fact in some dives Alcyone was completely dead).
Visibility underwater was rather low, shot with up to 1200 ISO in 35 meter depths (Dirty Rock, Alcyone). topside rainy every day. Did practice more on the "higher ISO - lower flash power - faster frames" shooting style and like it very much...
But still did enjoy it very much despite the excessive traveling going there. I think Cocos is still one of the top 10 places in the world.
A few images (ok, the tiger shot is rather low quality due to heavy cropping since far distance shot at low visibility and almost no flash lighting):