I'm considering shooting underwater in an indoor swimming pool.
It's not my pool, so I won't have easy access: maybe a few hours only and I think extra light will be helpfull.
Does any of you have some insights/experience with lighting the pool?
I'll have to use lights that will not be in the water at any time.

Does a 800W Arri create a 'lightbundle' or will it diffuse real fast without reaching far into the water?

Any thoughts/tips on positions of the lights?

Walter,
There are so many variables in your scenario.
You have to know more.
What is the size of your subject? Is it a person or 20? What's the size of the pool?
How deep you dive?
Do you plan to see the pool's walls? I had both scenarios with the walls covered in black and white,
We usually rent an underwater lightmeter.
You can try to put your lightmeter in an iPhone underwater pouch.
By the way, Arri 800 doesn't exist.
Follow this link:
ARRI Interactive Photometic Calculator (http://calc.arri.de/calculator)

Best,
Eugen

It's just 1 person taking a dive.
It has to look like a swimming pool: so showing the walls in no problem.
Offcourse light loses intensity the deeper you get; that's why I ask about the behaviour of light from a spot.

It's a 100m pool, but underwater I don't want to show all of it. Most important is the entry in the water.

A lightmeter won't be a very helpfull tool when I use a GoProHD ;-) : I have no control over the exposure and I can't tell what the camera will do. I just need to make sure it's light enough.

(I recall a friend of mine having a set of Arri lights with 800W bulbs. I could be mistaken.)

arri 800's do exist. it is arris equivalent of the redhead. i have a few in my kit and will be sorely
disapointed if they have ceased to exist...

for the pool i would use 1.2's or 2.5's but if it is exterior on a sunny day
and you were only using the entry into the water from underwater you could probably get
away without extra lighting.
placement of the lights would depend on camera position but i would favour back or edge
lighting.

the problem with tungstens is they would have no effect in daylight and getting them close enough to play without risking everyones safety.

cheers

THere are plenty of underwater lighting units available for hire in the US:

HydroFlex,Inc. --- Underwater Camera and Lighting Systems (http://www.hydroflex.com/)

Not sure what the equivalent if any would be in the Netherlands.

You are right Ian, there is an open face 800 Arri. I was thinking at Fresnel only.
I would also use a couple of 2,5 HMIs sideways and underwater fill if possible. Because is a very large pool I would place the jump spot in the corner in order to place the lights sideways as close to the shooting spot as possible.
If there's no other choice my advice is to use GO Pro Hero 2 and the new underwater housing. There is a huge difference in image quality and sensitivity between GP HD and Hero2. Also the regular GOPRO housing gives blured images and is not suitable for underwater photography.
Good luck!

If you want anything approaching a decent exposure/colour-accuracy on your subject - you'll need underwater lights.

Thanks Mark!
I'm not after true colors: I'm after the swimming underwater in swimmingpool look.
All blueish with light coming from above.

It's just testing stuff.
I'll probably rent a GoPro Hero 2 and a diving house.
(Thanks to this forum I know the normaal housing produces unfocused shots.)