The Sea and the Targaryens

The sea has been their element for centuries. Though never dragon writers, the Valerians had for centuries remained the oldest and closest allies of the Targaryens. The seam was their element, not the sky. That seems a pretty straightforward conclusion based on how that's written. It implies they were a naval powerhouse or at least navel-y oriented back in the time of Valerian. They probably didn't have the seahorse sigil. But remember, sigils and heraldry are more of a Westeros thing.