I contacted the artist who [designed][1] the coin pictured. Her name is Olga Drebas, and she is listed as one of the Interior Illustrators on page 2 of W:DH. In a [public tumblr post][2] she answered:

> The character on the golden coin is Ahghairon, the first Open Lord and founder of Waterdeep city, as per the art brief I had. 

For the record, Olga was very nice.


  [1]: https://olgadrebas.tumblr.com/post/178714776379/my-designs-of-assorted-waterdeep-coins-the-final
  [2]: https://olgadrebas.tumblr.com/post/182447731997/the-artwork-for-the-coins-you-did-is-beautiful