It looks like a nice addition to a backpack, light weight, compact, functional.
With a feathered arrow one can use it to hunt above water as well?
How to sell it?, well optimise the design, make it faster and cheaper to produce, yet easier, saver and more comfortable to use. Discover variations (such a reel?), offer different bands and spears. Do some research have people test prototypes, take notes for the guidebook. I think a guidebook can make all the difference, since this could be an affordable spearfishing starter kit, and these people do not know much about spearfishing at all, so giving them some knowledge about underwater hunting will greatly increase their chance of success, and appreciation of your product. Maybe even consider having a spearfishing guide writer teaming up for the project?
I think for beginners it's more about landing the smaller fish, so maybe a trident head needs to be included.
I'm just throwing out some ideas.
I feel inspired to try to make one myself.