Speargun Spears were introduced to RUST with the August 6th 2021 Underwater Update and are exclusively used with the Speargun to combat other players and sharks around shipwrecks whilst diving. Apart from the Torpedos used by submarines, speargun spears are the only underwater-specific ammunition in RUST.
While the weapon itself can be found quite commonly from several sources, speargun spears are obtainable only through researching and crafting them via the Tier 1 Workbench.
As eluded to, spearguns and thusly speargun spears are only able to fire while submerged underwater, rendering them completely useless for land-based combat. They function very much like the Crossbow when fired.