Yep, that too. And I've seen extraordinary measures taken to open a tagged & locked valve before... ("Gee, sir, I figured it was finished/left over/a mistake/forgotten from yesterday/got the date mixed up" etc ad nauseum)
But, this one sounds like a private yacht, and my bet is that the "norm" is a simple tag-out.
Either this boat wasn't tagged out, in which case the diver is largely responsibile for his own injury, or it was tagged out, in which case the tag-out was ignored.
I think it's about a 50/50 chance at this time, so I ain't willing to cast stones at either the diver or the boat owner ... yet...
Rick
"Private yacht?!?" Try Gloucester fishing boat. I seriously doubt the vessel is equipped for lock out.