Original, long question
was: "Why would it be honourable for a doctor not to try to take the
ship back, but it is honourable for a captain to lie and look like a
coward to his fellow shipmates by pretending to be a doctor when his
ship is overtaken? I see no honour in that."
An honourable fight would certainly include not fighting under false
pretexts (like under a false flag or betrayal by allied troops), but
would most certainly include using ruses, disguises, ambushes and
obfuscations.
Witness Aubrey disguising the "Surprise" as a whaler to lure the French
in, or employing a decoy to draw off the "Acheron", when it has the
dominating position in a fight. He even once (in the books) sails his
vastly weaker command "HMS Sophie" in under the guns of his soon to be
boarded enemy by flying the flag of a neutral country , but is_very_
careful to haul down the misleading colours before opening fire, having
now much less to fear from the enemy's broadside. By your standards
that would be 'dishonourable', but not from a millitary officer of that
age's point of view. They would call it "devious" and "circumspect." The technical term for such actions is "ruse de guerre," or "trick of war."
Besides, the French captain, disguised himself as a doctor simply because the
physicians/medical officers, by the nature of their profession would be
considered non-combatants, hence they would be allowed far greater
leeway/freedom even under the constraints of an honourable surrender,
because they would be expected not to harm anyone. Stephen Maturin
never enters into the fray of a boarding action while in service of the
Royal Navy - and only once at all, then in his function as the owner of
a privateer and an intelligence agent of the British Crown, not as a
physician.
And the sole doctor aboard a prize for Valparaiso would most certainly
be called upon to treat medical emergencies and inspect the healing
process of wounds for both sides aboard the prize, French or British,
because, under the hippocratic oath , they wouldn't raise a weapon or
cause undue pain to anyone under their care or supervision, a paciifism
usually extending even into other areas of life. Which " de Vigny" of
course is not affected by either...
It all boils down to _ personal_ honour. Once a (gentle-)man has
surrenderd and thereby given his word of honour to desist from hostile
action (until freed or exchanged as a prisoner of war ), he cannot
break this vow except by losing all and any respect as a person.
Which, very much, the French captain has not. He goes into "hiding" by
claiming to be the innocuous and lower ranked doctor, never having to
give his personal surrender that way, and as such, not endangering his
honour. Which is a ruse, so nothing defaming or nefarious about it,
from a viewpoint of "honour". It's simply clever and taking advantage
of a magnanimous attitude and assumptions by Aubrey.
Which in a way is wonderfully ironic, because Aubrey has just before
pulled the the wool over his eyes by disguising the "Surprise", now he
returns the favour by disguising as a 'harmless' doctor.
The tenets of honour you describe, 'standing tall and proud', 'upright
to the end' are pretty much along the line of chivalrous (medieval)
honour, but not 'gentlemanly' honour, which meant unvaryingly good
(polite and formally correct) manners, being a man of one's word,
integrity and loyalty to one's friends and country and being open about
it. Your set of proposed values would be described as "noble and old
fashioned, yet very foolish" by contemporaries of Aubrey.
And this is due to the fact, that "honour", per se, is not an absolute,
but varies markedly by culture, age and even social strata. The
"honour" of a 19th century New York ganger would vary tremendously from
that of 19th century gentleman or that of a samurai in the last days of
the Shogunate. And let's not even get started into the hodgepodge field
of "personal honour" (aka "integrity") and "machismo" ( aka
"pride/vanity" ), shall we ?