Weapons on the first island
Community write-ups treat the starting melee as two different tempos. Brass knuckles hit often and chip. The knife is slower but heavier. Either works if you only attack after a missed charge. Do not stand in front of the rush and trade.
The stun is the whole fight
The useful window is after a charge that misses. Sidestep, wait for the stun stars, then dump melee. Some players bait the crab into the lighthouse wall so the charge dies even earlier. That is a positioning trick, not a skip of the bait quest.
What you actually turn in
Walkthroughs disagree on the nickname (meat, shell, trophy). Keep the unusual piece and talk to the Keeper again. ShowGamer also has you talk to Melvin after that hand-in, then check the wrecked boat and the equipment board. A generic cooked crab does not open the boat. After the keys, buy the handheld Radar as a second purchase.