It's certainly frowned on by the BSAC! Scuba spearfishing can ruin a big bay quite quickly, and even snorkellers can do an awful lot of harm, especially as they tend to hit territorial fish like lumpsuckers and wrasse - WHILE THEY ARE GUARDING THEIR YOUNG!!!
Please don't, it's much more fun to watch them, and there's absolutely no credit in harpooning a fish that had no idea it was supposed to be scared of you.