To find it, do not go to Saronia immediately, keep the Enterprise!
The Sea Dragon is an enemy in Final Fantasy III, exclusive to the DS remake that can be easy to miss as it is only found in the seas of the Surface World. It is a slightly difficult enemy to defeat when it is encountered, but can still be easily defeated by using Lightning elemental weapons or the Thundara spell.
The Sea Dragon can be found anywhere on the Surface World waters after restoring the Water Crystal. It cannot be encountered after going to Saronia.