Said to observe more impartial mariner (8)
I believe the answer is:
seafarer
'mariner' is the definition.
(I know that seafarer is a type of navigator)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for seafarer that I've seen before include "man being attracted by waves?" , "Sinbad?" , "One on the water" , "Salt" , "Traveller over water" .)