He's only going snorkeling, he's not going to really need the bulk or the added headache of exposure with lights, unless he can dive really deep, but a first timer, doubtful.
The color red is complete gone at 12-ft deep -- 1/2 gone at 6-ft -- 1/4 gone at 3-ft. Yes, you need artificial lighting u/w for correct exposures, perhaps even more so with film. If you are floating at the surface and shooting something 6-ft under you, you won't have much red in the picture. I forget at what depth yellow is gone, it's next to go.
And yes, I have a lot of u/w photography experience. It's not something that can be covered in a single thread.
My recommendation is to start with protecting your equipment. Test it BEFORE you use it. Get a bucket of water, load up the housing and submerge it. Look for any water in the bottom of the housing, below the camera. My housings each had a 9v battery and small audio alarm. Two bare leads (+ and -) in the bottom of the housing, below the camera. One time I was at about 15ft on my way down and I heard the alarm -- surfaced and was able to fix the problem without any damage whatsoever to the camera or lens (Nikon). There was only a single drop of water in the house but that was enough to make contact between the two leads and sound the alarm. The problem was a piece of lent on a seal -- wicked sea water into the housing.
ETA: the greatest change in water pressure occurs closest to the surface. The deeper you go, the less variation. It matter because you need to consider dynamic pressure on the housing seals, not static pressure.