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It seems that the auto-off feature is triggered by the pre-flash. What happens if the camera's built-in flash is blocked completely in the housing? Would the focus light still be able to auto-off?
If you are shooting in any form of TTL (either hard wired or optical cable), then the light would still see a preflash from the UW strobe.
If shooting in manual strobe control, with the strobe set to either ignore a preflash or the camera does not even have one, well then the light would be trying to turn off as the strobe fires, and I don't have a clue how fast that would be and if it would leave any effect.
I think if that were the case, would suggest just getting a wide angle light, so that it does not show up as a spot.