Iso 800 film is for outdoor use, and isn't as sensitive as iso 100 film. That's why they put it in disposable waterproof cameras with no flash: They're for the beach or pool.
Got that backwards.
ISO 800 is much more sensative (and grainer) than ISO 100 film. It goes ISO 100, 200, 400, 800, 1600, etc. Each time you double the ISO, it takes 1 stop less of light to get the same exposure.
So if at 100 ISO the proper exposure is f/2.8 1/60th, for 200ISO the exposure wouldnow be f/4 1/60th or f/2.8 1/125th, and for ISO 800 f/2.8 1/500th. Or f/8 1/60th.
The put ISO 800 in most disposable cameras because they can have a faster shutter speed and smaller apeture resulting in more pictures coming out with more of the scene in focus, and less motion blur. If they put 100 ISO in the same camera they would have to use a larger apeture, or a longer exposure. The images could potentially be sharper and less grainy, but more people's pictures would have more motion blur, more shallow depth of field, etc.
They try to make the disposable cameras as idiot proof as possible, and sacrifice image quality.
Kodak tried marketing Ektar 25 aka Royal Gold 25 which was a great fine grained film. The problem was people would load it in the point and shoot cameras and their built in flash was only good to 5-10 feet, their shutter speeds were slower, etc, so you had people getting more under exposed and blurry pictures. For those who know photography 25 speed film was great for certain things, like scenery shots, etc.
However, it wasn't working out for the masses, so Kodak shifted gears and went with the ISO 800 aka MAX, ZOOM MAX, etc, which when used in people's small point and shoot cameras resulted in extended flash range, faster shutter speeds, greater depth of field, which meant the average picture for the masses was better, but grainier, but in a 4x6 print it was acceptable.
So the under water cameras are set up with a fixed exposure and apeture, and even though the ISO 800 is sensative film, the exposure isn't long enough for the P3D. However, if you had the P3D on TURBO and had it within a couple feet of something, that maybe enough, but if you're shinning it on something 10-20 feet away, probably not going to cut it.
Update:
Just did a test with my light meter. I think I did this right. The sunny 16 rule for photography is, if the subject is being hit with direct full sunlight, the exposure it approx f16 and the shutter speed = the film's ISO.
So if you have 800 speed film, f1/6 and 1/800th of a second. If your camera doens't have 1/800th, you can go 1/500th and you'd just slightly over exposure, which the negative can handle, or go 1/1000th and be slightly under exposed. Usually it's better to slightly over expose.
So I set my light meter to f/16 and ISO 800. Now I just had to move my P3D Q5 back and forth from the sensor until I got about 1/800th of a second. I found I had to be around 6-8 inches from the sensor, on turbo, to get this exposure.
You could try taking shots at 1 to 2 feet, but the image will probably be under exposed, IF the camera is set up for this exposure. It maybe set up to slightly over exposure in full sun, so if it is cloudy out it will still get something.
So anything past 1-2 feet would be very under exposed.
