With a great avatar comes great misidentification
We pronounce Boatswain as "boat-sun" but we'd never call a Boatswains Mate just "boat-sun" because it would sound to similar to Bosun (where the "t" is noticeably dropped in pronunciation). Boatswain Mates are either called Boatswain Mates, "boats" or "BMs" for short. A Bosun is simply called Bosun.Interesting ... do you pronounce them the same?
I was under the impression, that Bosun was just the "idle" spelling of Boatswain (or the spelling for those who have heard the word, but may never seen it written in full), much like Gunwale is sometime seen written as Gunnel. Maybe things evolved differently on your side of the puddle.