I saw the following graph in a blog post about how shipping noise affects whales. Why does the sound seem to propagate better in deep water than in shallow water? Are there any known marine species that changed their preferred depth due to this effect?
What you're seeing there is an effect of the ocean thermocline. The speed of sound changes rapidly in the upper few hundred meters of the ocean from a combination of temperature and pressure changes (sound moves faster in warmer water, and in higher-pressure water). This produces a natural waveguide called the SOFAR channel, which can carry sounds for thousands of kilometers.
It's suspected that fin whales use this channel for communication.
Additional to Mark's answer mentioning the sound speed profile and SOFAR channel, there are effects in shallow water that increase transmission loss, especially for lower frequencies:
In shallow water sediment interaction of the sound will often be the dominant source of sound attenuation (for anything but very hard sediments) (see: https://dosits.org/science/advanced-topics/shallow-water-propagation/ & https://dosits.org/science/advanced-topics/how-does-sound-travel-in-very-shallow-waters/).
This attenuation is frequency dependent, but generally the seafloor acts like a highpass filter, causing low frequencies (which dominate ship noise and long range noise) to be absorbed (see: https://www.researchgate.net/publication/343987334_Accounting_for_sea_floor_properties_in_the_assessment_of_underwater_noise_radiated_from_ships_in_shallow_water for good introduction and model).
In general frequencies ~< 200 Hz are absorbed and dissipated in shallow water, and higher frequencies attenuate due to volume scattering and heat loss leaving the dominant noise in shallower waters between several hundred Hz and a few kHz.
In deeper waters, with no mechanism for low frequency attenuation, low frequency noise can travel very far (even in the absence of a SOFAR channel) given the negligible attenuation loss (see: http://resource.npl.co.uk/acoustics/techguides/seaabsorption/).