Protection against water left beside the ocean (4)
I believe the answer is:
seal
'protection against water' is the definition.
(as in sealing a leak)
'left beside the ocean' is the wordplay.
'left' becomes 'l' (common abbreviation).
'beside' is a charade indicator (letters next to each other) (I've seen this in other clues).
'the ocean' becomes 'sea' ('sea' can be a synonym of 'ocean').
'l' put after 'sea' is 'SEAL'.
(Other definitions for seal that I've seen before include "Imprinted disk affixed to a document" , "Stick down" , "Stamp of approval" , "Insignia for sale" , "Amphibious fish-eating sea mammal" .)