How do harpoons work?

Question

from what ive heard bullets lose most of their speed after travelling a few metres under water.

if this is the case why are harpoon guns able to shoot underwater

Answer 1

relative to a bullet, a harpoon has a much larger length to cross-section.

so a harpoon slices a small hole in the water and has a lot of momentum to back it up.  a bullet slices a comparatively large hole in the water and has little momentum to back it up.

Answer 2

harpoon guns shoot through the water to catchh fishys. or you can spear them. but dont, coz its cruel.

Answer 3

Harpoons are horribly cruel things, fish have feelings too.