Was it an Assault Speargun?
..But seriously... I taught my kids to shoot and insist that ALL guns and shooting weapons are treated with the same margin of safety. That includes Bows, Paintball Markers, BB/ airsoft guns..
-Always treat them as if they are loaded and pointed in a safe direction. (usually either straight up or down)
-Never put finger on trigger unless you ready to fire
-Always make sure that downrange is clear and safe
I have never been spearfishing, so I don't know what safety rules are recommended or enforced. I would hope there was some serious guidelines for this activity if they are going to be in the water with me.