Here are convenient sources of information. One is Wikipedia, https://en.wikipedia.org/wiki/Water_distribution_on_Earth . Use its references where possible, not Wikipedia itself ( see also https://en.wikipedia.org/wiki/Wikipedia:List_of_citogenesis_incidents ).
Volume of Water
Each of the 7.5 billion people alive in 2017 would have as their share: 184,578,505.8 cubic metres of all water, fresh and salt: oceans, icecaps and glaciers, lakes, rivers, groundwater, and water in the atmosphere; 1,411,639.4 cubic metres of fresh liquid water in the ground, lakes, swamps, and rivers; 12,400.2 cubic metres of fresh-water lakes and rivers; 1,717.9 cubic metres of water in the atmosphere, mainly as water vapour; and 4,160.9 cubic metres of water in icecaps and glaciers.
For 10 billion people, this changes to 138,600,000, 1,060,000, 9,311.3, 1,290, and 3,124.4 cubic metres.
And for 12.5 billion, it changes to 110,880,000, 848,000, 7,449.04, 1,032, and 2,499.52 cubic metres.
This comes from the data below, which depicts water as spheres showing:
(1) All water, fresh and salt: oceans, icecaps and glaciers, lakes, rivers, groundwater, and water in the atmosphere. The largest sphere over western U.S., 1,385 kilometers in diameter, 1,386 million km3 in volume.
(2) Fresh liquid water in the ground, lakes, swamps, and rivers. The mid-sized sphere over Kentucky, 272.8 kilometers in diameter, 10.6 million km3 in volume. This includes water deep in the ground, inaccessible to humans.
(3) Fresh-water lakes and rivers. The smallest sphere over Georgia (below Kentucky), 56.2 kilometers in diameter, 93,113 km3 in volume.
(4) Water in the atmosphere, mainly as water vapour. Not shown in the picture, but would make a sphere 29 kilometers in diameter, or 12,900 km3 in volume.
(5) Water in icecaps and glaciers. Also not shown, but would make a sphere 390.8 kilometers in diameter, 31,244 km3 in volume.
[ There’s a nice chart on https://www.usgs.gov/special-topic/water-science-school/science/where-earths-water?qt-science_center_objects=0#qt-science_center_objects . Unfortunately, unlike the picture above, it’s not copyright USGS, whose own images are in the public domain: it comes from a textbook. But I could mimic it. Wikipedia, https://en.wikipedia.org/wiki/Water_distribution_on_Earth#/media/File:Earth’s_water_distribution.svg , thinks it is public domain, but I think they’ve got that wrong. ]
Surface Area of Water
Each of the 7.5 billion people alive in 2017 would have as their share 102.94 square metres of river surfce and 48,195.5 square metres of ocean surface.
For 10 billion people, this changes to 77.3 and 36,190 square metres.
And for 12.5 billion, it changes to 61.84 and 28,952 square metres.
This data comes from:
1. Rivers: 773,000 square kilometres. This is from https://pubmed.ncbi.nlm.nih.gov/29954985/ by George allen and Tamlin Pavelsky. It updates previous estimates, multiplying them by roughly 44%.
2. Oceans: 361,900,000 square kilometres. From https://www.ngdc.noaa.gov/mgg/global/etopo1_ocean_volumes.html .