Historically, how has the fraction of Earth covered by water changed?
Can't say much about the later epochs, but the 'hellish Hadean' epoch without oceans is kind of an outdated idea.
We have zircon records indicating that crust and oceanic formation was already done to some unknown extent 4.4 Gyrs ago. The zircon analyzed in the nature paper given originates from interaction with sub-crustal material in a watery environment at this date.