Your DBs are being shot down by the natural AA gun of the ship you're diving. Your pilots Bomber ability provides defense against this gun, so raising the number of experts and vets as well as simply leveling them will help them survive. They can reach a state of immunity to these guns by lvl 90.
When manual dive bombing, I find that zooming in on my target helps me aim and if you use the technique of holding the drop button to prevent them from diving at all, you'll find the bombs land about a FFs length behind of where you aimed, so try to compensate for that either by aiming slightly ahead of where you want the bombs to land, or attack along the length of the enemy ship.
