You can get a Sea Dragon for free, though it is very rare being a crown pet. It drops from bosses in Celestia; Astraeus, Mithraya, and Ptolemos. You can also buy it from the crown shop for 12,500 crowns. There is also a small chance of getting it from the Dragon's Hoard pack and finally, you can hatch with somebody with a chance of getting the Sea Dragon, but it won't have every first generation pet ability.