Softly dancing in dressy pink clothes, they've no costumes (6-7)

'they've no costumes' is the definition.
(naked swimmers without swimming costumes)

'softly dancing in dressy pink clothes' is the wordplay.
'softly' becomes 'p'.
'dancing' indicates anagramming the letters.
'clothes' means one lot of letters goes inside another.
'in'+'dressy'+'pink'='indressypink'
'indressypink' with letters rearranged gives 'skinnydipers'.
'p' placed into 'skinnydipers' is 'SKINNY-DIPPERS'.