Swimming in sea, dog was distressed (8)
I believe the answer is:
agonised
'distressed' is the definition.
('distressed'->'agonized' is in my internal thesaurus and 'ize'->'ise')
'swimming in sea dog' is the wordplay.
'swimming' is an anagram indicator.
'in'+'sea'+'dog'='inseadog'
'inseadog' anagrammed gives 'AGONISED'.
'was' is the link.
(Other definitions for agonised that I've seen before include "suffered torment" , "Dog in sea (anag) - suffered anguish" , "Felt mental anguish (over a decision)" , "Was in great doubt" , "Tormented" .)