The words "sea lice" mean very different things to some people, because any bug in the ocean is often referred to as a sea louse.
In the context of salmon and fish farms, the term refers to parasitic copepods that hitch a ride on the fish and that eat mucus and skin of their host.
With older fishermen, the term often refers to carnivorous amphipods that can strip a halibut carcass overnite. The reason they are more active at night is due to the potential for predation on then during good visibility (as you pointed out).
These "sea lice" are very different critters, so the amphipods are probably not more "prevalent" under salmon cages (although shrimp and prawns may be - if they remain unaffected by slice in fish faeces and salmon pellets from above).
The parasitic copepods found on wild and farmed salmon (i.e. Lepeotherius salmonis and Caligus spp.) reproduce by letting go eggs which shortly hatch into a series of very small, free-swimming stages, nearly invisible to the naked eye. These stages float closer to the surface for some time, and are not found as swarms under the cages like you describe for the amphipods.
