Being very thin, decline to expose oneself in the water (6-3)
I believe the answer is:
skinny-dip
'expose oneself in the water' is the definition.
(skinny-dipping is swimming in the nude)
'being very thin decline' is the wordplay.
'being very thin' becomes 'skinny' (I've seen this before).
'decline' becomes 'dip' (both can mean to fall or droop).
'skinny'+'dip'='SKINNY-DIP'
'to' is the link.
(Other definitions for skinny-dip that I've seen before include "Bathe in the nude" , "Swim in a birthday suit" , "Swim naked" , "Swim in the nude" , "Mainly North American expression meaning to bathe naked" .)