One paddles in ocean, swimming first (8)
I believe the answer is:
canoeist
'one paddles' is the definition.
'ocean swimming first' is the wordplay.
'swimming' indicates anagramming the letters.
'first' becomes 'ist'.
'ocean' with letters rearranged gives 'canoe'.
'canoe'+'ist'='CANOEIST'
'in' is the link.
(Other definitions for canoeist that I've seen before include "rower" , "Boat user" , "sportsman" , "Paddler" , "Person who paddles a kayak or suchlike" .)