Since human bone are much stronger, could you break your tip?
You could also try to cut the weight belt with a power head
Seriously, i dont think, with the range (3-5 m) of a speargun that you could have to shoot someone with it to save his/her life.
I would also change, "if you cant reach me, spear me" to "if you cant reach me, dont dive with me".
Just my 2 Euro cent.
