A closer look reveals other fun gems. There's more oxygen than hydrogen in seawater, even though the chemical formula for water is H2O, so pure water should contain twice as much hydrogen as oxygen. The ocean, however, has lots of dissolved oxygen gas. Fish would not survive otherwise. Meanwhile, magnesium is the most common component in seawater after H, O, Na, and Cl, because it's common in the Earth's crust and it dissolves easily in water.