The smartest idea I have seen lately for GoPro video was what one of the guys from the ScubaNation.com TV show was using to supplement the video they were shooting with their big monster Gates system..... He had a short polespear, the really short kind, and at the end of it, he had fitted a holder on to the gopro...
The way he shoots is letting the goPro hang from the bottom, as it is steadier, and is weighted to "want" to be at the bottom....which means that the video is all shot upside down....however, with the GoPro software, you can do an almost instant "flip" to make all the video right side up..
The polespear has enough length to add a long polar moment of inertia to a tiny little camera...with just the goprop by itself, you will accidentally move your wrist a tiny bit, and get a big jump in the video-- a shake...with the short polespear, the vertical and horizontal is much slower, more like with a big pro camera.
It really does work.
The downside of the helmet mount is that you will be constantly panning left and right and up and down, looking for the right shots....you can get them this way, but you will waste a lot of footage you will just have to delete out....The polespear approach also lets you get the camer nicely far back into some ledges you might see cool stuff in...
Your friend is absolutely right about the need for the diffuser...and an opaque plastic bag could be a smart way to fix a light with a spot/focused beam--at least make it much better.....this is kind of related to a top side video guy getting some Home Depot flood lights, and covering them with wax paper to diffuse them--and ending up with video lights almost comparable to a $4000 system for less than $75..
"Almost"
You want maximum diffusion, and you only care about trying to light up the area about 1 to 2 , or 3 feet at most, in front of you..This is true even with a $4000 video light system.
Here is how my video lights work in a wreck penetration...ie., pretty dark area :
way-too-much-fun - YouTube
p.s.
the other videographer that comes in about half way into this video does a few things I don't think video or photo guys should do....
Like many of even the most famous photographers, this guy decides the only thing that is important, is the shot, and he stands on the bottom to shoot... He happens to be one of the strongest divers you could ever dive with, and a great shooter, but in this moment, he got so excited he did not care about anything but the perfect shot
.. In this case, it did no harm, and did not silt too much, but in most instances, it would silt, and "could" damage the bottom....Just a little work on maintaining a flat horizontal body position and maintaining perfect bouancy with breathing, and it is easy to stay in the mid water collum as you can see I did. When I shoot in a wreck, the water stays clear...I can't tell you how many shooters crawl around like billy goats, and destroy viz within minutes !!!
The other thing is you can see his light is way more powerful than needed, and is almost like an explosion of white.... You don't need all that much horsepower in your lights....but you do need good diffusion, as your friend suggested.