Seaside feature: sex initiation plus nude swimming (4,4)
I believe the answer is:
sand dune
'seaside feature' is the definition.
(sand dunes may be found by the seaside)
'sex initiation plus nude swimming' is the wordplay.
'initiation' indicates taking the first letters.
'plus' becomes 'and' (I've seen this in another clue).
'swimming' indicates an anagram.
The first letter of 'sex' is 's'.
'nude' with letters rearranged gives 'dune'.
's'+'and'+'dune'='SAND-DUNE'
(Other definitions for sand dune that I've seen before include "Small hill by the seaside" , "Ridge created by the wind in deserts and near oceans" , "Small hill beside the sea" .)