So, a mate and I had completed the entire voyage - dug up both the chest and the key and cracked open the chest. We made way for the isle where Capt Brigsy was to await the delivery and upon reaching her she was ethereal (green and ghostlike, unlike normal where she is in full colour). She wouldn't accept the skull - also the waves of skeletons absolutely wouldn't relent spawning all over. I'm unsure if that was what causes her to go ethereal but it made it impossible to cash in the skull of sirensong and thus we had to vote to cancel the voyage after trying for over an hour.
Kind of a huge let down, all things considered, that we did as is intended and couldn't reap our just rewards.
Another touch of feedback for the Skull of Sirensong event as a whole --- what is the point of trying to run the Skull if the opposing player crews can simply scuttle and the game spawns them between you and the isle where Brigsy awaits? It quite literally puts this event back into old Skull Fort camping territory where it's not feasible to complete when folks can just camp the cash in point and always be there before you simply by voting to scuttle to setup shop.
