Concrete can be placed underwater, but it has to be relatively still water. The concrete is placed through a tremie, sort of like a flexible elephant trunk of thick rubber. That way the concrete won't be in movement with water breaking it up. Instead, the concrete is below the water and expands as more flows into the mass.
Rushing water over it with a down hill run would bust it up prior to set, IMHO.
The baskets are an imitation of a gabion basket: 
The idea is to have this sort of rip-rap held stable by wire or other baskets long enough for quick setting concrete to be placed on top or to otherwise hold together as a barrier.