Last year my wife and I flew to Sulawesi from the UK with Singapore Airlines to Singapore and Silkair to Manado, when we booked the flight we asked for extra baggage allowance for divers and were instantly given an extra 10kg each, no problem. (Although Singapore Air don't advertise this on their website, Silkair do).
All dive related publications I have read state that the allowance is available, but you must phone to ask whilst confirming your booking.
I have just booked flights for us both to travel the same route and when I contacted SA to confirm and get the extra allowance I was told that this was an old policy that no longer applies. Their new policy states that I could book an extra 15 kilos between us but would have to pay for the first 6kg at £42 per kilo each way, the extra 9 is free (Silkair's policy hasn't changed so no need to pay any extra from Singapore to Manado and back).
I managed to get a free 5kg each by convincing the lady that I was a Brad Pitt double and promised to take her out for lunch in return for a waiver. Boy is she gonna be disapointed
So if you're planning to fly your gear with Singapore Air check what you can get away with before booking.
It seems that the Airline Industry hates divers.
Nige.