Hey Kris,
I have no experience with harpoons but have had lots of exposure to fish spears used on walleye spawning beds in Northern Ontario.
You usually see factory-made, steel, multi-pronged, barbed spearheads with broomhandle/hockey stick shafts or handles cut from trees on-site. Like these
http://www.sbtoutdoors.com/fish_spears.htm I have also seen some very crude homemade versions (e.g. pitchforks, nails-poles) but they all kind of follow this idea.
Occasionally you see improvised wooden spears created on-site similar to these
http://wildwoodsurvival.com/survival/fishing/spears/spears01.htmlOnce I saw a snare-pole used for fishing, being a thin wire snare (not unlike what you would use for a rabbit) on the end of a long pole.
A snagger also works well when fish are concentrated during spawning, it could be a large hook on the end of a pole or even on the end of a piece of stiff wire with a wooden T-handle.
When spawning in shallows fish are very vulnerable and often can be harvested in a survival situation with a hoop net, rock, club or even your bare hands.
I think it is well known in the preparedness community that methods which are usually prohibited are the most effective ones in a survival situation.
I have no exposure to barracudas but I did have a young child get very freaked out last summer over a large snapping turtle. The child went into murky water at the beach to wash off "body glitter" and the big turtle surfaced right in front of her. I figure the turtle thought the sparkling flashes and movement were a large school of baitfish.
So even away from the ocean, during certain times of the year on spawning sites, a spearhead/harpoon could be a useful food gathering tool in a survival situation (check your local regulations concerning possession/use of these items in your area).
Mike