These shallow waters upset sailing boat (5)
I believe the answer is:
pools
'these shallow waters' is the definition.
(pool can mean a small body of water)
'upset sailing boat' is the wordplay.
'upset' shows that the letters should be reversed in order.
'sailing boat' becomes 'sloop' (type of sailing boat).
'sloop' in reverse letter order is 'POOLS'.
(Other definitions for pools that I've seen before include "still waters" , "Small areas of still water" , "See 1 down" .)