It's a little bit complicated. Sea levels at the moment are actually relatively low on a long term scale, but on a shorter term scale, we're in an interglacial, so they're relatively high. For example, when there was more water locked up in the polar ice caps over the last few million years you had land bridges that allowed passage across the bering strait and whatnot, However if you look past, I dunno, 10 or 20 million years you're almost always significantly higher. The peak value is debatable, it's easily passed 100m higher than now (that's probably close to the long term average), but figuring out the absolute highest values are complicated by tectonic changes in the heights of land surfaces and stuff, and erosion and whatnot. So giving an estimate for the total amount of oceanic at any given time is difficult - I'm not even willing to hazard a guess, really, but I can say with certainty it's never hit 100% in the last 500 million years (since we have contiguous evidence for land based life from that time).