I believe the phrase is indeed from a song, called 『さよならの夏』, which is probably recognisable by ear to many on this forum, as it was popularised by/featured in the Studio Ghibli film 『コクリコ坂から』 ("From Up on Poppy Hill").
As you will hear in this cover (go to 2m30s for the relevant section), the lyric is「そら」not「から」, so the meaning of 「そら」should be reflected in your translation.
The meaning of 「そら」is, as you say, most straight-fowardly 'sky' -- and that meaning seems to broadly fit with what's going on with the other lyrics, which mention boats and flying, amongst other things -- so "sea of sky" or "sky-sea" or similar seems OK to me, in terms of the lyric's plain interpretation.
Whether this represents a 'correct translation' in practice is going to be down to your personal judgement as the translator, that will likely depend on a variety of factors (e.g. Are you publishing your translation in some form? If so, is it sung, or just appearing on paper? Does the lyric need to rhyme? Or have particular intonation to match the music as it is sung? Who's the audience? How poetic do you want to get? etc.).