.... and clothes. .... toiletries.... I take the same stuff - I am just very careful to remove bit I don't need. I even don't take the integrated wieght pouches from my BC to save a bit!
Alex
Alex, don't forget the standard Cousteau red speedos!
... it may be cheaper to get an extra ticket! I suggest asking the cute receptionist at work if she/he wants a free trip......
SI, that's so dirty old man in south east asia sounding. 
Maybe you should distribute that letter with the British UWP as I find it as a good "weapon" against complicates checking people...

Yes disseminate. I use to have a written letter from SIA allowing me extra quantities of luggage. Then like Pmooney, I achieved a certain FF status, so for awhile they stopped bugging me. Only flying the puddlejumpers in domestic airlines do I run into occasionaly problems, but by then each kg is about $3 so I put it down to the cost of the flight.
One thing I do recommend for those who want to is to carry the housing yourself. Nothing more frustrating than to arrive without checked luggage which include your camera and housing. Everything else from dive gear to underwear can be bought or rented at low expense, but not a housing and camera. At least this way you can still shoot.
I have 3 different hand carry configurations:
1. PITA check-in countries (eg. Australia, HK, UK etc)
- Heavy photovest carrying cameras, lenses and accessories, housing with UWA lens protector in hand, with camera inside, laptop bag with BU drive and batteries and chargers etc.
2. Non-PITA check-in countries (eg US,SIN etc)
- Photo-laptop backpack + fullsize roller case (usually Porter Case(WARNING: it doesn't fit in the Size-Wise box)) plus housing in hand.
3. Puddle jumper planes/crowded domestic flights:
- Photovest, photo-laptop backpack and housing in hand.
In most cases, once they see a big housing, they usually don't mess with you. It pays to have photojournalist credentials (press card or when I forgot mine, a Canon Professional Services member card (with pic) worked
)
Suitcases:
Since I carry most of the camera gear it's down to dive gear and video lights.
I always have my NorthFace waterproof Duffel bag for clothes and wetsuit. It has a dry zipper and can take a beating. It acts as a wetbag on boats and carries luggage when you're travelling. It's also as light as a normal duffel bag. I'm worried as I can't find them anymore. 
I have a big 32" cordura suitcase with hard sides and bottom but soft top. I wrap my lights in clothes or wetsuits and strap them down. Fits spare chargers, dive kit and other sundries (as well as the standard Cousteau speedo suit, I bring several though and rather not live in one the whole trip like they did on the Calypso and Halcyon(and yes I have confirmation of this from several former crew members!)) and weighs in under 25kg all the time.
Another bag I have is the NorthFace large roller duffel. It's very water resistant and has reinforced rear and rigid bottom so anything fragile can be packed there with clothes as cushions. Even with heavier cold weather clothes and booties and fins, usually drops in at 28kg.
Sometimes you just have to say "uncle" and beg the airline in advance for overweight. I did this with several airlines and now they even have the letter ready for me.
Happy light travels and may the bags be with you.