"I've read that it has been much hotter in Earth's prehistoric past. It was so hot that the oceans steamed contantly, and Earth's climate everywhere was unsuitable to mammals, unless they were very small, nocturnal, and lived in underground burrows."

I don't think it was this hot, the average ocean temperature millions of years ago was around 4 degrees warmer- in other words instead deep water being close to freezing as it is today, it was 4 degrees warmer and global temperature being around +10 degree warmer than present.
So oceans wouldn't be steaming- unless you refering to a local affect due to some impactor or volcanic event- or global affect due very large impactors [mass extinction events] and all this would continue only for few decades or centuries- a blip in the long history.

"The icecaps are receding, whether we speed it up or not by burning fossil fuels. Is being steam-boiled the future for humanity? Or will we find a way to air condition our entire existence without dumping heat into the oceans and atmosphere?"

Let me make this simple, if tomorrow for whatever reason average global temperature was to increase by 10 degrees [far more then even wack jobs are "predicting"] , then it will take centuries at this higher temperature to melt the ice caps and even longer to have a significant affect on ocean temperature. Or stack all this ice on the land at the Equator and it will take centuries to melt- in other words effectively increasing it's average local temperature by +20 C and it still takes centuries to melt all of it. If we had some mad desire to melt all this ice [say for whatever reason it was very profitable to melt it] we could not do this in less then 100 year time period [assuming there was a limit upon the number nukes we used to less than 10,000- the environment impact report limits it to this:)- and say less than half the world resources/wealth/capital were used in this project]