I found that death from below task is quite hard to get if you liberated the islands. I could not do it from water because pirates allways heard when I swim. This solution worked for me:
1. go to am 12 base and accept wanted task from bilboard (x:688.0, y:786.5)
2. approach from x:669.5,y:783.4, target is on 2nd floor (use camera for taging)
3. sneek and kill other pirates with sniper and leave your target alive
4. there's a ledge you can clime just above side door in the ground level floor
5. you have to lure your target to the ledge with rock and then do DEATH FROM BELOW kick :-)