Agree map at sea would show strength of current (8)
I believe the answer is:
amperage
'strength of current' is the definition.
(I've seen this before)
'agree map at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'agree'+'map'='agreemap'
'agreemap' anagrammed gives 'AMPERAGE'.
'would show' acts as a link.
(Other definitions for amperage that I've seen before include "Strength of current" , "current measure" .)