[egiboy]

I wouldn't go to Webster's for definitive information about hydrography. The definitive source is "Limits of Oceans and Seas" by the International Hydrology Organization.

The 3rd edition of that document, which came out in 1953, lists four oceans - same as Webster's sans the Antarctic: https://iho.int/uploads/user/pubs/standards/s-23/S-23_Ed3_19...

The 4th edition of the document, which has been in draft since 1986, adds the Southern Ocean: https://legacy.iho.int/mtg_docs/com_wg/S-23WG/S-23WG_Misc/Dr...