Long topic, but yes, the oceans are inhabited by merfolk and sahuagin nations. It's touched on in the Stormreach sourcebook.

— Keith Baker (@HellcowKeith) August 26, 2017

Thank you. I’m writing a campaign that takes place in Stormhome. I was also wondering if ships needed to be registered with a government.If you're docking in a Thronehold nation. Also, "Lyrandar" would count as a government for this purpose.

— Keith Baker (@HellcowKeith) August 27, 2017