How long does light last say for continuous use in night dives?
TG-5 does not have a continuous light; it has a flash. A flash pulses for less than a millisecond at a time, which can be used for still photographs, but not for video, or to light your way in the darkness. Additionally, on-camera flashes have two problems that make them unsuitable for underwater use:
1. They're weak. Underwater, you're not dealing with lack of light; you're dealing with the fact that the natural light that you do get is strongly tinted towards blue (or green, in freshwater). This happens because water naturally absorbs red and yellow light, and the deeper you dive, the less red you have left from the natural sunlight spectrum. At 30m, red is pretty much gone completely, and you have to bring it with you. Regardless of depth, in order to render good color, your flash (or, as they're usually referred to, in order to avoid confusion with flashlights, your strobe) needs to not just light the subject, but overwhelm the blue-tinted natural light. Counter-intuitively, this takes a lot more light on day dives than on night dives, since at night, you're not competing with the sun for color spectrum.
2. They produce backscatter. Unlike land photography/videography, when you dive, you almost always deal with particulates that are suspended in the water, be it silt, plankton or whatever. When your light source is placed right next to the lens, as is inevitable for an on-camera flash or LED, these small particles in the water reflect its light right back into the lens - this is called backscatter, and it manifests as small bright dots all over your image. In order to avoid it, you need to shine your light, regardless of whether it is a continuous source or a strobe, from off-axis to your lenses line of sight. You can appreciate this effect even without a camera - next time you go on a night dive in water that is not totally clear, try holding your torch right next to your face, then stretch your arm sideways and compare.
Together, these two factors result in on-camera flashes being useful underwater only at very, very short distances - single centimeters, i.e. an inch or two in US-speak. This close, the light they emit does not yet get a chance to dissipate into uselessness, and the lens/flash parallax is large enough to avoid backscatter coming into view. Any further and you start having problems.
Here is a very minor example of backscatter - I had off-camera strobes, but because I was shooting a fisheye lens with a very wide field of view, some of their light ended up in line with the lens and bounced back. Note the many bright dots on the bottom right of the image:
https://i.imgur.com/IB4nHaN.jpg
Edit: I think I misunderstood your post - if you were referring to the Sea Dragon 2500 light that SeaLife bundles with their Micro 2.0 camera, it is rated for 1 hour burn time at full power.
How deep can sea life go without a housing for water proofing?
SeaLife DC2000 is rated to 18m without a housing, 60m with a housing. SeaLife Micro 2.0 is permanently sealed and is rated to 60m by itself. DC1200, DC1400, etc, are discontinued - the only cameras that SeaLife sells right now are DC2000 and Micro 2.0.
