Regarding "calm" vs "cold" in the Peggy Seeger / Calum MacColl song, I reckon the only way to know for sure is to ask the author(s).

But besides finding the absence of a hard stop in the way the word is pronounced in the recording, and accounting for Peggy's trans-atlantic accent, the main reason for going with "calm" is poetic. Of course the waters were cold, that's too easy. A sinking in calm waters creates dramatic tension to add impact to the points that the writers make in the second part of the song.