For the boatman, the ocean is rough trouble primarily (8)
I believe the answer is:
canoeist
'for the boatman' is the definition.
(canoeist is a kind of boatman)
'the ocean is rough trouble primarily' is the wordplay.
'rough' indicates anagramming the letters.
'primarily' suggests taking the first letters.
The initial letter of 'trouble' is 't'.
'ocean'+'is'='oceanis'
'oceanis' anagrammed gives 'canoeis'.
'canoeis'+'t'='CANOEIST'
(Other definitions for canoeist that I've seen before include "Paddler" , "Person who paddles a kayak or suchlike" , "rower" , "Boat user" , "One using light boat" .)