Figure 1: Left: Earth with oceans and atmosphere represented as spheres (Ice-blue, atmosphere; Cyan-blue, water/oceans). Right Center: Venus with atmosphere; Far Right: Ceres compared to Earth and Venus in 50 kilometers per pixel.
In the above image, on the left, the radius of Earth = 6378000 m so volume of Earth = 1.08678129 × 10^21 m^3. Average depth of ocean over 3/4 of Earth's surface = 3800 m so it over the whole surface it would form a hollow sphere 2850 m thick. Subtract Earth's volume from that of the larger sphere to get a volume for the water of 1.45753101 × 10^18 m^3. The radius of a sphere of that volume would be 703358 m, a little over 1/10th the radius of the planet, and represented by the clear cyan blue sphere. The adjacent ice-blue sphere represents the volume of the atmosphere. The Hydrosphere is the layer of water which covers about 71% of the earth's surface. The average depth of the oceans is 3794 m (12,447 ft), more than five times the average height of the continents. The mass of the oceans is approximately 1.35 quintillion (1.35 × 10^18) metric tons.
The center right image is that of Venus, that was taken by the European Space Agency orbiter Venus Express. With Earth and Venus approximately the same size, and having formed at the same time, astronomers believe that both planets likely began with similar amounts of water due to comets during the Late Heavy Bombardment that ended 3.9 Billion years ago. However, Anabar (2009) et.al, page 4, indicates: “by contrast there appear to be no surfaces on Venus that date back to the early bombardment.” The presentation of Sizemore (2004) that “Venus has undergone a catastrophic, global resurfacing event in recent geological history” that apparently “ended 700-800 Million years ago” and due to a global recycling of the planetary crust because of volcanism.
Despite Venus being called Earth's "twin", its surface conditions are far from being alike to our home planet's. Venus's surface is surrounded by a thick mass of clouds. The atmosphere of Venus is heavier than the atmosphere of any other planet. It is made up of carbon dioxide, small amounts of nitrogen and water vapor, and very little portions of argon, neon, sulfur dioxide, and carbon monoxide. The atmospheric pressure on Venus is about ninety times more than it is on Earth. It is about 1,323 pounds per square inch. If one were to stand on Venus, the atmospheric pressure would crush you within seconds. The surface of Venus is very hot and dry. Moreover, there is no liquid water because it would boil away from the heat. Most of Venus (65%) is covered by flat plains, where there are thousands of volcanoes. Thirty-five percent of Venus is made up of mountains. The highest is Maxwell, which is seven miles high. There is also a canyon that is .6 of a mile deep. Another feature of Venus is impact craters, which are formed from an asteroid and a planet crashing. There are two large highland areas: Ishtar Terra and Aphrodite Terra. Coronae, another characteristic of Venus, are circular volcanic structures surrounded by ridges, grooves, and lines. Arachnoids are another unique feature to Venus. Arachnoids are circular and oval features with concentric rings and a group of fractures
As Venus and the Earth are comparable in size, the inclusion of the atmosphere within the image would be representative of the Earth with an atmosphere (which is confined in the ice-blue sphere on the left side of the image and above the Earth). Directly to the right of Venus, the grey sphere represents Ceres. The Planetary Society presents a topic on Ceres and the aspects of a potential “ocean” by stating:
Exactly where the layers lie inside Ceres depends on how much ice it contains, which depends on how dense its rocky component is. If Ceres is less icy, it has a relatively thin water ice layer of about 70 kilometers (45 miles) in thickness; if Ceres is more icy, its ice layer would be about 120 kilometers (75 miles) thick.
There is an excellent image of Venus with oceans that was created by in Australia with a few interesting concepts of how the planet might look.
It is widely accepted that the current dryness of the Venus atmosphere is the result of extensive evolutionary processes. The amount of carbon in the form of CO2 in the Venusian atmosphere is comparable to the best estimates of the Earth’s carbon inventory, which is largely locked up in carbonate rocks. This finding suggests that a “runaway” greenhouse scenario led to the lack of plate tectonics and biogeochemical cycling on Venus. According to this hypothesis, the primordial inventories of volatile elements on Venus and the Earth were similar (on a mass-adjusted basis); the present differences in distribution between atmosphere and lithosphere are evolved.
If so, the extreme scarcity of H2O in Venus’ atmosphere could be a consequence of photolysis of primordial H2O followed by loss of H to space, possibly within the first billion years. The high D/H ratio of the Venus atmosphere supports this hypothesis, but this interpretation is complicated by the fact that volatiles can be accreted long after formation – even in the geologically recent past – in the form of cometary impacts, and by uncertainties in the D and H escape fluxes. Hence, the D/H observations could alternatively be a result of H2O escape and resupply in the last billion years.
Since the average depth of the oceans is 3794 m (12,447 ft) on the Earth, roughly equivalent to 2.4 miles deep. The water layer proposed for Ceres, while smaller in circumference, is many miles thicker. The total volume of water on Earth is about 1.4 billion cubic kilometers, around 41 million of which is fresh water. If Ceres' mantle accounts for 25 percent of the asteroid's mass, that would translate to an upper limit of 200 million cubic kilometers of water.
If Ceres could eventually be destabilized from the current orbit and impact Venus, the resulting ocean depth would range 1/7 of that on the Earth or 542 m. Unfortunately, due to the pressure and the temperatures the water would become vapor and would remain in the upper atmosphere and then the hydrogen would be stripped by the solar wind and lost to space.
It would take approximately 10-20 Ceres size objects diverted to Venus to recreate enough water vapor in the atmosphere before atmospheric destabilization occurred with rain starting to fall on the high upland mountains. If smaller comets were diverted or Kuipler Belt objects were brought into the inner solar system, then a much larger number >10,000 would be required. A series of impacts, might assist in the removal of a significant part of the current atmosphere by blasting it into space. This would facilitate lower atmospheric pressures from the current atmosphere pressure to a lower.
NOTE: The future is a unknown progression of humanity and development of technology and innovations, maybe over the next 900 years humanity might decide that Terra-forming could occur. How orbital dynamics of Ceres could be change and the technology involved is the decisions of the future generations of humanity. Maybe, Ceres could impact Mars, given the amount of water that exist on Mars and on Ceres a planetary impact would create something on the order of magnitude of a Hellas Impact basin and liberate the water from both Mars or Ceres to form a northern ocean. With terrain being ejected material from the impact event and that might create enough heat to start the convection within the mantel and subsequent magnetosphere on Mars. This is a thought exercise and conceptual idea, a seed in the garden for the future generations of humanity and the unknown technologies that occurs hereafter.
Anbar, A. D. (2009) et. al. Astrobiology Research Priorities for Mercury, Venus, Earth, and the Moon. A White Paper for the 2009-2011 Planetary Science Decadel Survey . Arizona State University.