An article at CNET reports that "four billion years ago, Mars had enough water to cover the entire planet with an ocean between 100 and 1,500 meters (328 and 4,920 feet) deep, and that between 30% and 99% of that water is now trapped in minerals in the crust."
A quick visit to Mr. Google led me to an EarthSky article suggesting the same history for Earth:
"... most of Earth’s water was on the surface at that time, during the Archean Eon between 2.5 and 4 billion years ago, with much less in the mantle. The planet’s surface may have been virtually completely covered by water, with no land masses at all... These new results not only provide a glimpse of what Earth used to be like as a water world, but also have implications for other water worlds in our solar system such as Europa, Enceladus and other ocean moons. Those moons are different from Earth, however, in that their global oceans are covered by crusts of ice. In many ways they are similar to the ice-covered ocean environments at Earth’s poles.There are several such ocean moons known in our solar system. Even some dwarf planets like Ceres and Pluto had subsurface oceans and may still today. With thousands of exoplanets being discovered, and estimated to be in the billions in our galaxy alone, how many moons are out there? Likely more than we can easily count right now, and if our solar system is any indication, many of those moons may also be ocean worlds.
Science extends the speculation re the origins of life:
The evidence for larger oceans challenges scenarios for how life began on Earth, says Thomas Carell, a biochemist at Ludwig Maximilian University of Munich. Some researchers believe it began at nutrient-rich hydrothermal vents in the ocean, whereas others favor shallow ponds on dry land, which would have frequently evaporated, creating a concentrated bath of chemicals.A larger ocean exacerbates the biggest strike against the underwater scenario: that the ocean itself would have diluted any nascent biomolecules to insignificance. But by drowning most land, it also complicates the thin pond scenario. Carell, a pond advocate, says in light of the new paper, he is now considering a different birthplace for life: sheltered, watery pockets within oceanic rocks that broke the surface in volcanic seamounts.
What amazes me is that all this water would have had to come from bombardment by water-rich asteroids and comets.