The original fish spears were wood and only wood. Tips were fire hardened and lasted long enough to catch fish over several days of fishing. I think I'd try that first before risking my knife. Worst case I can always carve a new spear if the tip softens or breaks. I haven't tried spearfishing but in my experience even pine ( very soft wood) stays pretty hard if all you are doing is occasionally dipping it in the water. It will soften if you leave it in though.

- Eric