Video answer: Muffled/underwater audio effect - hitfilm pro tutorial
Top best answers to the question «Why does my recording sound underwater»
Your Audio Source is Too Close, Too Far or Off-Axis
The problem with being too close to a device while recording is that sound can overwhelm the capsule in a condenser mic and lead to a muffled, distorted recording.
Video answer: The magic of audacity - how to fix muffled/poor quality audio