(HULK)
(Smash)
You can just make a tidal trap. Guy did it on the Naked Castaway and it looked pretty simple.Spear fishing is very difficult.Knife doubles as a cutting implement for making rope, shelter etc. Also, attach to end of a stick and you have a fishing spear.
Step 1: Find a tidal pool that fish can swim into. Bait it if possible.
Step 2: Wait for fishes to swim into it.
Step 3: Drop a bunch of rocks into the route into the tidal pool so the fishes can't swim out.
Step 4: Wait for tide to go out.
Step 5: Collect fishes