21.1°C is the global surface temperature of the oceans.
By the way, that is a record. The oceans have never been as hot before 2022 and not this hot for so many days as it was in August.
Source: https://www.faz.net/aktuell/wirtschaft/klima-nachhaltigkeit/neue-rekordhitze-in-den-weltmeeren-19126745.html#:~:text=Weltmeere%20so%20warm%20wie%20noch%20nie&text=Die%20Oberflächentemperatur%20der%20Ozeane%20hat,bis%202022%20niemals%20erreicht%20wurde.