Pretty boring answer, but the ancient name of the Pacific Ocean was just Sea 海 or Eastern Sea 東海.

The word 洋 has another earlier meaning of "vast" or "great" or "plentiful". However initially it is believed to originally be the name of a river, and only later did it get extended by the semantic connection to water to mean "ocean". Prior to that, the default "big body of water" word was simply 海 (hence we also have 上海 and 下海 as the original Jiangnan fishing settlements, not 上洋 and 下洋).

The use of 洋 for the ocean, and specifically 太平洋, happened much later, around the 16th century.

This meaning of "vast" or "broad" comes up in early texts like the Shuowen Jiezi 說文解字 as well as the Shijing 詩經. We have numerous historical records of 洋 as a modifier on words having nothing at all to do with water, such as virtue or other positive qualities, but also in reference to things like river.

Note also this kind of use of things like 海 to refer to the Ocean was common for geographic features. The Yangtze River was originally known simply as 江, and the Yellow River as 河. Rivers more generally were called 水, or other semantically related words.

So, anyway, back to the original question, at least for Chinese sources the answer is pretty dull. They simply called it the Sea/Ocean, since it was essentially the only one that mattered or was known. Once there became greater need for distinguishing different oceans, well by then we had 太平洋 so it was a non-issue.

Hope that helped. Sorry it's not something more interesting. Let me know if you have any follow ups.