Hi:
I thought about this scenario also. What I thought I would do is:
Check the local camera rental shop and see what your camera rents for. (My Nikon D2x goes for $250.00 CAN a day with a discount of 7 days for a 5-day payment). I would do the same for any lenses they request from you. I would charge those prices, as they seem to be the going rate. You also have to realize that you won’t have the use of your camera for the time the client has it. If you make your livelihood, or part of it, with your camera, then this is down time for you where you will be unable to generate any income.
The amount I paid for my housing with ports and extension rings if fairly close to what I paid for my camera body. Work out a percentage cost for the principal outlays for the camera and all the housing equipment and charge accordingly.
If my underwater housing cost 75% of what I paid for my camera body, then I would charge 75% of the camera rental price or $187.50 (camera rents for $250.00 a day) for the housing and ports, with the same discount for a week’s rental. I would also do the same for any underwater strobes and sync cords requested. Don’t forget memory cards cost money too.
Finally, if the dive is not too technically oriented, I might offer cameraman services and be able to keep an eye on the equipment and also get some free diving out of it to boot.

You might want to offer a further discount, as you would be able to minimize the risks to your equipment this way.
While others may feel you should charge more for the underwater equipment because of its’ specialty and scarce nature, the above should put you in the ball park for a reasonable amount to charge.
Now, if you are really brave, you could try offering camera and underwater housing insurance for say 10–15% of the total rental price by self insuring the equipment. i.e. You take the money, hope everything goes well and that you get a dry working camera when the client returns, otherwise the client gives you a soggy digital camera and housing and walks away without any responsibility for pooching the camera -- at this point the insurance premium will turn into a down payment for another camera and possibly underwater housing. You have to have very good nerves and really pray that the client doesn’t pooch your camera and housing while it is in their care to be able to do this though.
Let us know how it goes.
Paul Walker.
Nikon D2x, D7000, Aquatica D2x, AD7000, SunStrobe 200 x2, Inon Z240 x2, TLC Arms