What is the best way of translating 'at sea'? For example, "The sailors fought bravely at sea". The translation 'in mari' seems the closest to me, as opposed to the literal 'ad mare' and treatment like a locative with just 'mari'. Is 'in mari' the best/most common way of translating it? Does there exist an idiomatic expression for this?
It seems in mari would be a suitable translation of the English “at sea.”
For example, M. Tullius Cicero wrote,1
vix in ipsis tectis et oppidis frigus infirma valetudine vitatur, nedum in mari et via sit facile abesse ab iniuria temporis.
which Evelyn Shuckburgh translated into English as,2
Even in houses and towns it is difficult to avoid cold when one is unwell, to say nothing of escaping the inclemency of the weather at sea and on the road.
M. Tullius Cicero. The Correspondence of M. Tullius Cicero, Arranged according to Its Chronological Order. Ed. Purser, Louis Claude; Tyrrell, Robert Yelverton. Vol. 4. London: Longmans, 1894.
M. Tullius Cicero. The Letters of Cicero. Trans. Shuckburgh, Evelyn. Vol. 2. London: Bell, 1905.
Therefore, I believe that the best choice for your purposes would be in mari, because the sentence is describing a place where. Also note how the locative would not be used for the specific place construction in this context because mare is not one of the listed "special" forms (right column).