The USGS has produced a graphic that shows the Earth stripped of all water – salt, fresh, clouds, water vapor – with all the water rolled up into a sphere:
How much water is there on, in, and above the Earth?
The ball of water is about 860 miles in diameter.
What the resultant illustration looks like is a dry unterraformed planet with a small ice moon from further out. The sci-fi implications should be obvious… if you find an Earth-sized world that doesn’t have enough water (like, say, Venus) and you want to make it a water world… you need to transport an iceball about that big. Of course, *crashing* an iceball that big into the planet would likely be a pretty bad thing; much of the ice would get slung back out into space. But putting it into a low orbit about the world and then chopping it apart and throwing chunks at the planet might be a good way to go about it.
I leave it to the student as an exercise to determine how to move a moon of water 860 miles in diameter.
