To reproduce: Get in a boat
Float near the shore at night until a mob notices you (I used zombies for their superior tracking abilities)
Move a generous distance out to sea, away from any possible shore sounds
Wait for the mobs to swim out to you
As they approach, you will hear the usual mob sounds and footsteps even if the mobs are in deep water.

What I expected to hear:
Just mob sounds and splashing noises, no footsteps.

Other way to reproduce

• switch to creative
• go into the ocean
• sink to the ground
• spawn a cow
• cow will make splashing sounds and the normal walking sound