1. Outgassing from the Earth's interior: This theory suggests that water was trapped within the Earth's mantle during its formation and was later released through volcanic eruptions and hydrothermal vents. This is supported by the presence of water vapor in volcanic emissions and the high concentration of deuterium (a heavier isotope of hydrogen) in Earth's water compared to water in comets.

2. Delivery by comets and asteroids: This theory proposes that water was brought to Earth from outer space by impacts from comets and asteroids. Comets are known to be rich in water ice, and some asteroids also contain water. This theory is supported by the presence of organic molecules in comets and the similar isotopic composition of water in Earth's oceans and in some comets.

It's important to note that both of these origins likely played a role in the formation of Earth's oceans. The relative contributions of each remain a subject of ongoing research and debate.