@MyLittleDragon: The Pirate Dragon is impossible to breed using Elemental Dragons (Sea Dragon + Dark Dragon; or Dark Dragon + Sea Dragon -- the order doesn't matter), BUT it is possible to breed a Sea Hybrid with a Dark Hybrid. The only way to get a Pirate Dragon or a Petroleum Dragon is by breeding a Sea Hybrid with a Dark Hybrid. You cannot get a Pirate Dragon by breeding with a Sea Dragon + Dark Hybrid, nor with a Sea Hybrid + Dark Dragon any more, because of the recent changes in the breeding system.
Pirate is dificulty 2. If you want to breed Pirate effectively, you should use Zombie Dragon (Dark secondary) + Any Sea Hybrid => gives no dificulty 2 dragons but Pirate. The key of getting Pirate is making sure of total rate is about 50-54, no higher or lower. Then the chance will be 40% for every combination
Example: Zombie + Waterfall, Seashell, Ice Cream, Icecube, Mud, Mercury
Levels are adjusted to have the total rate of 50-54
Petroleum + Petroleum = 40% chance too, but only if you have them