To swim said to be the alternative to this depression (4)
I believe the answer is:
sink
'depression' is the definition.
(I know that sink is a type of depression)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for sink that I've seen before include "Scene of iniquity or washing up" , "place of iniquity?" , "Basin for washing up etc" , "Disappear below the surface" , "Descend into water" .)