I dont know, but you could have an aquatic road called the nautilbahn, but you have to be swimming quite fast when you enter the submerge.
If the city is enclosed by a large bi-valve you could call it shell-ter.
OP could have a tax collector that moonlights as a buccaneer. Call him Pontius Pirate.
First thing that came to mind for a city of mermen - Ethel.
Balthasar, after the original myrrh-man.