To breed this Terra/Sea hybrid, you will first require your parent dragons to be level 10 or higher. They can either be hybrids or legendary dragons. However, for the best chance to get it, try the following combinations:
Remember, they both have to level 10 or higher.
Sea + Terra
Nenufar + Chameleon
Ice Cube + Cactus
Mud + Mercury
Neon + Cactus
Wind (15) + Petroleum (15)
Pure Flame (20) + Hedgehog (20)
Source: Internet