The speaker has a sudden, slightly dizzy thought: the ocean isn't just water, it's mostly the remains of dead things, piled up over an almost unimaginable stretch of time. The odd, mock-scientific figure of 4.54 x 10⁹ years (roughly the age of the Earth) "+ 1%" gives the idea a deadpan, offhand tone, as if the speaker is doing a quick, slightly absurd sum.
In the second stanza the thought becomes an image. The ocean is a graveyard, and its waves are "gleaming grave" markers that crash and melt into gray. That colour spreads outward: gray beaches, gray people, gray skies. Everything seems to take on the dullness of death. The mood is bleak but also a little wry. The casual title and the opening "that feeling when" sound like internet talk, which sits oddly beside such heavy subject matter. That clash suggests a mind trying to joke its way through a realization that is genuinely unsettling.