The following by
auv-ee explains in laymans language the affect of water temperature on acoustic channelling.
For horizontal isotherms (temperature changes with depth but not laterally; the usual case considered), there is no bending of the straight-down ray. As the angle of incidence increases away from the vertical, bending will occur, either toward or away from the vertical.
Surface waters vary around the world, but are warm in the tropics. Thus, near the top of the water column, sound speed is elevated, due to increased temperature, and at great depth (1000s of meters) the temperature is uniformly cold, but the sound speed increases with depth due to increasing pressure. In the mid water (1000-2000m) the sound speed is minimum. This effect creates the deep sound channel, where near-horizontally directed sound is trapped and guided.
The critical angles vary with the particular profile of sound velocity vs. depth at any given place and time, but generally, sound can be exchanged between two points that are shallow and deep over a cone exceeding 45deg from the vertical. If this were not so, transponder navigation of surface vessels and acoustic communication with deep systems would not be possible. In fact sound leaving either a surface or deep source in tropical water will both be deflected toward the vertical as it approaches the mid-water channel. Things are different in the polar regions where the surface water is so cold that channelling occurs at the surface, rather than deep.