I thought they just cause paralysis that causes people to drown because they can’t surface in time
They cause paralysis, yes, but they paralyse the diaphragm too so you can’t breathe without help, above or below water. Hence the drowning being most common, but yeah, not always fatal. However, they can also cause heart failure and blindness, so this is still pretty stupid. Though they only bite if you pester them enough, they tend to prefer to run away.