Assuming the astronomy speculation is correct on the wiki
. Are there any suitably placed basins that could be Cuegan's sea? No one but Randall can know how much of the Earth's water will be in the polar ice caps in the Randallverse in Cuegan's time. So describe where sea level would need to be for your basin to flood with ocean water.
Eleven thousand years provides plenty of times for volcanoes and ice ages to sculpt a planet to form new basins. Even without major events, the Niagara Falls will become the Niagara Rapids once it erodes away the Niagara Escarpment.
I disagree about your last statement, eleven thousand years is just enough to get into the next ice age, except that global warming has almost certainly forced that one to be skipped. And it's not nearly enough time for geological processes like volcanoes or continental drift to have any significant effect. Eleven thousand years is enough time for significant amount of ice to either form or to melt, depending on where our climate actually goes from here, and melting is more likely by far. So I think we can significantly raise or slightly lower global sea levels, but apply no other geological transformations.
So where does that leave us? Check out the Western Interior Seaway from the late cretaceous: https://en.wikipedia.org/wiki/Western_Interior_Seaway
. If we melt most of the ice in Antarctica and Greenland, which seems like a probable result after a few centuries of global warming, we could bring something like this back.
Looking at the map, how about we choose a point at the tip of southern Illinois, where the seaway forms a bit of a peak jutting into the heartland. Why there? Because I think there would be extremely weak tides at that location. Sitting at the northern tip of a shallow body of water, there would be little room for the water to slosh to the East or West, which is what forms the tides. We might be on the edge of the Mississippi or Ohio river valleys, which could mean a couple hundred feet of elevation change before reaching a large flat plain.
Of course, this puts our pair at a body of water that connects to the whole ocean. And even with the worst case hysterical predictions of global warming, I think we're going to be measuring sea level rise in inches per year, not inches per week.
Ok, so what could cause an extremely rapid rise in world ocean levels? I was about to say I can't think of anything, but then I did. The thing about the massive glaciers that rest on Antarctica is that many of them are grounded below sea level. And by 'many', I mean most of western Antarctica. The scariest global warming scenarios is that warm water is able to get between the ice and the rock and cause accelerated melting from below. As things exist now, these ice sheets are blocking more glaciers, ones sitting on higher elevation rock, from descending downhill and reaching the sea. But if you melt the blocking ice from below, it suddenly floats free and can get out of the way. The combined effect is that a ridiculous amount of land ice is able to rapidly flow into the sea, which is where rapid sea level rise will come from. We've already experienced huge ice sheets breaking up in a matter of weeks, though these are smaller external ice sheets rather than the thicker ones in the core of the continent (Larson B sheet collapse in 2002).
So, could a single massive ice sheet collapse event raise world ocean levels at a rate of inches per day over several days? I suspect not, but it might be interesting to get a more detailed analysis, one with actual math involved.