Fish as a club-like weapon?
No. Just no. You already have two answers which make this clear.
Fish as a weapon at all?
Perhaps you could trick them (I leave to the reader how to trick them) into eating some fugu. This is that puffer fish variety that if very carefully prepared by an expert chef who trains for years, is considered a delicacy. Do it just slightly wrong and one bite can kill 30 men. So of course, let your newbies prepare the enemy's fugu.
Ah, but your enemy is not so easily tricked. Well, you could very carefully hurl Lion Fish and Rock Fish at them. However, this might not be enough in an all-out combat situation. As National Geographic tells us, “It [the venom] won’t kill you, but it’ll make you wish you were dead”.
You want your enemy all dead, not just wishing dead. One last try then. Take the parts of the fish and re-purpose them. Real-world primitive peoples have used shark teeth, bones (including fish bones), and swordfish bills as components of tools and weapons for thousands of years. This bends your rules quite a bit, but it's the only feasible chance you've got.