The guy in this video clip is wearing a G-string which under normal circumstances would be regarded as extremely provocative:
https://www.youtube.com/watch?v=CivphkeEHM8
A glance through other videos of these Christmas swims show other example of males wearing thongs/mankinis.
It would appear that because there is a fancy dress option it is acceptable to where this type of costume, because it is seen as 'humorous'.
Curiously, there appears to be no examples of women wearing thongs/G-strings - perhaps because to them these are not humorous, and conversely a situation like this for a women would be seen as provocative.