What does the "water" refer to in Wallace's talk?

Wallace uses the metaphor of water to illustrate his belief that people often lose sight of life’s most important truths simply because those truths are so omnipresent. He compares this dynamic to a fish not knowing what water is simply because the water is ubiquitous and taken for granted. Wallace warns that essential truths, such as the value of compassion, often seem like banal platitudes simply because of how ubiquitous that truth is in life.