I would imagine its a situation of missed shot, spear travel beyond visibility. I can not imagine it was mistaken identity and heaven forbid it was deliberate, most spearos have a deep passion for the sea and to spear a seal is taboo, regardless of laws, the law obviously being that it is illegal to use speargun's on seals. Although I believe it was an accident, it still totally outrages me, firstly and most importantly because of the suffering of the seal and secondly, this is a totally avoidable accident, if you cant see for the total length of potential spear travel, don't take the damn shot!!
Pound to a penny, if this is you, you know its you, do the right thing, fess up and get in touch with Steve, you cant undo the damage but you can help make it better