With a dome diffuser such as the ones for SEACAM strobes one can cover the near field of a fisheye lens. I have thousands of shots done this way with one strobe mounted at the 12 O'clock position. I have also shot the same fish eye lens with an Inon strobe with the Inon flat diffuser installed - earlier 220 model that is the same size as the 240 models. The corners come out dark unlike SEACAM. My conclusion is that dome diffusers are superior to flat ones at least for fisheye lenses. Adam H tested one a while back but good luck finding the test on this site.
I have shot a lot of salmon behavior in streams some of it after sunset - see my web site esp under "salmonid topics" for behavior examples. However I am a stills shooter. I use the same cameras, housings, focusing lights, and strobes that I use for scuba diving. It is all done by remote control using SEACAM remote control components http://seacam.com/en...s/remote-system
I set things up wearing fishing waders then work the cameras from an elevated shore whenever possible - for a better view. Otherwise I will stand on gravel bars in the stream. Depending on current strength and camera setup (e.g. dome size) I use up to 30# in ankle weights to help hold things down. Even with this much weight when the males fight they can knock the camera around requiring a re-set. For this reason I do not recommend leaving your gear alone. When there is very little current sediment can end up on the dome port from digging and fighting also requiring entering the stream for wiping off. As well I will swap out strobe and focusing light batteries while the setup is in situ. Chest pockets are useful for this! It may take 2 to 3 strobe batteries in one day to get to the spawning if it even happens before I have to leave. At open canopy sites where and when the sun shines I do available light shooting at high ISO so don't use strobes for all my shots.
If I was to do video I would start using something similar except I would need a housing upgrade as my 1Dx does not AF in video but my 1Dx2 does. AF is important when shooting at point blank range as is typical. I would next think about an external battery for the camera since most of my still shoots go on for many hours - even the big pro batteries in a 1D will not last long (great for stills but doubtful for video in a housing). Possibly would use lights that could run off a generator or separate car battery. The batteries would be on land and run to the camera setup with waterproof cables.
IR does not go far in water so not sure if that will even work. You might consider super sensitive (to light) cameras aka night vision. You might have to add a few zeros to your budget. None of this stuff comes cheap.
I did some dives there back in 1994 before heading out to the Revillagigedo Islands on the Baja Treasure. I recall boarding diving skiffs right in the CSL harbor for the local dives. As well the operator did a day trip using their van up to Cabo Pulmo. There was a lot of current on the first Pulmo dive so that many did not do the second one which had no current. All this was quite a while ago. I do not recall the diver operator. I shot my underwater photos on that trip with a Nikonos RS and film!