Hunting..Underwater...Using Water??

Recently I have acquired some bamboo plywood that Ive been using on various projects, tables and such, but lately have been wanting to craft a speargun. The bamboo is more or less the body which I am confident I can build. However, the mechanics of a speargun are decieving me. Ive only used hawaiian slings prior to this project so the trigger mechanism has been my focus. I found an old pump air rifle I had, while harvesting the parts I initially wanted, I wondered of the possibility of using the pump to fire the spear. I researched it and found hydro pneumatic spearguns and aquatech devices. My question then becomes, what modifications would need to be made in order to make an operable speargun? Would the water thats being "compressed" work in an air pump while underwater? Not sure about any of this and may swith to bands altogether, just curious.