Considerable stretch of water - you'll need the oars then (5,3)
I believe the answer is:
north sea
'considerable stretch of water' is the definition.
'North Sea' can be an answer for 'water' (North Sea is an example). I'm not sure about the 'considerable stretch of' bit.
'you'll need the oars then' is the wordplay.
'oars then' can be anagrammed to 'NORTH SEA'.
Though, I'm unsure how this is indicated.
Can you help me to learn more?
(Other definitions for north sea that I've seen before include "Source of UK's oil/gas" , "Waters east of the UK" , "large body of water" , "UK source of natural gas" , "(Formerly) German Ocean" .)