By going round with some mariners, one sees the sea (5)
I believe the answer is:
briny
'the sea' is the definition.
('briny' can be a synonym of 'sea')
'by going round with some mariners one' is the wordplay.
I cannot quite see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
'mariners' could be 'rn' (Royal Navy) and 'rn' is present in the remaining letters.
'by' is within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'sees' is the link.
Can you help me to learn more?
(Other definitions for briny that I've seen before include "Of salty water" , "Salty; (the) sea" , "The main" .)