Boatman's Boatman, you say? I don't agree! (2,3)
I believe the answer is:
my eye
'i don't agree' is the definition.
(expression of disbelief)
'boatman's boatman you say?' is the wordplay.
'boatman's' becomes 'my' (where the crossword setter is 'Boatman').
'boatman' becomes 'I' (where the crossword setter is 'Boatman').
'you say?' shows a homophone (sound like).
'i' sounds like 'eye'.
'my'+'eye'='MY EYE'
(Other definitions for my eye that I've seen before include "What rubbish!" , "the detective I hired?" , "expression of disagreement" , "rhyme" , "bunk?" .)