Underwater laser as motion sensor

I was wondering if anyone on here could help me, i'm trying to create a camera trap for underwater use. Traditional camera traps use infrared motion sensors to fire a camera and strobes when an animal breaks the plane. Because of obvious differences between the way light travels in water versus air, no one has been able to effectively create a way to trigger a camera in water. This is further complicated by the fact that many non-mammal marine sea creatures (ie fish, crustaceans etc) can see ultraviolet light (not sure of the exact wavelengths but could find out) so they could potentially be scared off by a UV triggering devices. Does anyone know of a low powered laser device (can't harm fish and battery life is critical) that would be suitable for underwater use as a triggering device? Essentially when the fish breaks the plane of the laser it would send a signal to fire the camera. Details of the waterproofing are already worked out but were stuck on the underwater motion sensing problem. What spectrum/color of laser would be most appropriate and what device can you use to send an extremely low electrical signal to fire the camera once the plane is broken? The detection field between laser and sensor needs only to be a few yards in water pressure as deep as 120 fsw.
This has never been done in a non-military application so it would be quite ground-breaking.
Any suggestions are greatly appreciated.
Matt

Use near infrared instead of mid/far infrared. That travels through water just fine and very few (if any?) animals can see it. That will obviously use power though. Might be better to use changes in ambient light to trigger the device instead of creating your own.