It was officially confirmed by the Russian defence department this morning. The official version: it sunk while it was being towed due to a storm (spoiler: there were no storms in the black sea at that time.)
My version: it sunk right away and they lied to save the face.
I looked at the nautical charts for Black sea[0]. It looks like the area near Odessa is quite shallow (less than 100m (~300ft) deep). I'll hazard that it was a total loss from a get go, but they towed it just far enough to be able to let it sink to deeper waters (like 1000+ m deep (~ 3000ft)).
My version: it sunk right away and they lied to save the face.