FWIW, I would have dropped the gun and kept the flag and the other divers.
You can always buy another gun, and it might even be covered by insurance, but if the float takes off down-current with the boat happily following it, while you're trying to get your spear unstuck, and a bunch of other people are drifting on by, things could have turned out a lot worse than just losing a speargun.
At the very least, I think the captain would just about have a heart-attack after following the float for who knows how many miles, only to find that there are no divers under it.
Terry