The positive gradient in the deep isothermal region is solely due to the pressure effect. In the summer, the seasonal layer tends to have a strongly negative gradient, for the same reason as given to explain the diurnal variation. In winter, the water is generally warmer than the air. A lot of heat is lost through advection and radiation. However, one would not expect to see cold water setting on top of warm water for very long.
Convection brings the warm water to the surface destroying the effect.
The surface layer tends to be closer to isothermal than anything else. Additionally, strong winter storms and their large waves frequently mix the surface layer to a depth of up to 100 m. For the nearly isothermal surface layer, one could expect a weakly positive gradient above the main thermocline. The change of propagation speed with depth will manifest itself through refraction of the sound. A graphical method of illustrating the effects is called ray tracing.