I haven’t used a Jon line before so maybe I’m wrong, but I always thought they were for clipping off to an anchor or mooring line? OP was holding onto the reel end of a DSMB. A Jon line would’ve effectively just added a few more feet of line, no? Not sure how that would help a diver maintain a safety stop depth if this specific instance. But I might be missing something, so please let me know if I’m wrong.
same here