http://www.surveyorslexicon.com/index.php?Search=&RefNum=742... :

mate: an assistant to the captain

----
Grand Robert & Collins (www.lerobert.com):

mate1 [meɪt]

e Brit /Merchant Navy/ : ≈ second m (capitaine • m)

/US Navy/ ▶ maître m (dans la marine)

See: first
----
◆ first mate noun [on ship] : second m

◆ first officer noun [on ship]
= first mate

----
(Grand Robert de la langue française)

second, onde [s(ə)gɔ̃, ɔ̃d] adj. et n.

ÉTYM. V. 1155; secunt (v. 1119), secund (v. 1138); lat. secundus « suivant », rac. sequi « suivre ».

II N.
A
2 N. m. (1587). Personne qui aide qqn (en se conformant à ses vues, ses désirs).
➙ Adjoint, 2. aide, allié, appui, assesseur, assistant, auxiliaire, bras (droit), collaborateur, lieutenant.

Spécialt. Mar. Officier* du bord qui commande immédiatement après le commandant.
Grade de second.

----
Et pour faire bonne mesure:

http://www.photographers1.com/Sailing/NauticalTerms&Nomencla... :

First Mate - The Second in command of a ship

---
http://en.wikipedia.org/wiki/First_mate :

"A Chief Mate (C/M) or Chief Officer, usually also synonymous with the First Mate or First Officer (except on passenger liners, which often carry both), is a licensed member and head of the deck department of a merchant ship. The chief mate is customarily a watchstander and is in charge of the ship's cargo and deck crew.[1] The chief mate is responsible to the Captain for the safety and security of the ship. Responsibilities include the crew's welfare and training in areas such as safety, firefighting, search and rescue."

-----
=> Dans aucun dictionnaire, glossaire ou lexique, général ou spécialisé, "mate" n'est traduit par "matelot", c'est pourquoi je demandais une référence à Cyril B. Se baser sur l'âge uniquement me paraît un peu mince comme preuve. Il faudrait davantage de justification. Ce n'est pas parce que "mate" veut aussi dire "camarade, copain, aide", qu'il veut dire "matelot" dans la marine!

Voici toute l'entrée "mate" dans le Grand Robert & Collins (et les Anglais sont des marins par définition!):

mate1 [meɪt] → synonyms
1 noun
a [at work] ▶ camarade mf
b Brit ✰ “friend” ▶ copain ✰ m, ▶ copine ✰ f
■ he's a good mate : c'est un bon copain
■ cheers mate! ✰✰ : merci, mon vieux ! ✰
See: classmate, playmate, workmate
c “assistant” ▶ aide mf
■ plumber's mate : aide-plombier m
d of animal ▶ mâle m, ▶ femelle f
✰ humorous; of human; “spouse etc” ▶ compagnon m, ▶ compagne f
e Brit /Merchant Navy/ : ≈ second m (capitaine • m)
/US Navy/ ▶ maître m (dans la marine)
See: first

--------------------------------------------------
Note added at 23 hrs (2012-05-20 09:06:40 GMT)
--------------------------------------------------

Et à l'inverse (FR > EN), dans le Grand Robert & Collins:

matelot [mat(ə)lo] nom masculin

a ***[en général] « marin » ▶ sailor, ▶ seaman***

[dans la marine de guerre] ▶ ordinary rating Brit, ▶ seaman recruit US

■ matelot de première/deuxième/troisième classe : leading/able/ordinary seaman

■ matelot breveté : able rating Brit | seaman apprentice US

b « navire »
■ matelot d'avant/d'arrière : (next) ship ahead/astern

It must be noted that in the field of sport fishing, an 'offshore mate' has a very specific rôle, quite different from that of a 'mate' in the Merchant Navy, for example.

Note that here, 'offshore' refers to the type of fishing carried out, and opposes 'inshore' — this has nothing to do with being at sea or on land, nor does it necessarily relate to 'offshore' in the sense of the oil industry.

Here's a typical example of this type of work described by someone seeking a job:

http://www.sportfishermen.com/board/f188/offshore-inshore-ma...

--------------------------------------------------
Note added at 23 hrs (2012-05-20 09:33:18 GMT)
--------------------------------------------------

In informal language, especially in family yachting, with small crews, 'mate' often just means a helper or assistant; traditionally, the head of the household is the 'Skipper' and his wife the 'Mate' — of course this is just casual, colloquial usage; but the use of 'mate' in the current context seems more akin to this sort of usage than the 'official' Merchant Navy officer status.

--------------------------------------------------
Note added at 1 day5 hrs (2012-05-20 15:30:31 GMT)
--------------------------------------------------

Maybe not quite, Asker. It would be risky to use that, as many of the refs. on the 'Net make a distinction betwene a specifically fishing assistant who helps with bait, lines, etc., and a general mate, how helps with hospitality etc. So although they are indeed an 'assistant' for 'pêche hauturière', it might perhaps be better to try to avoid the potential ambiguity of that word order, leading one to initially read it as 'assistant de pêche'