I had a city on a narrow strip of land between the ocean and a lake.

I built a harbour, but it was placed on the lake side, i.e. where my ships couldn't reach it.

I'm not sure what would be the best solution for this; maybe where ships can traverse the city tile, or else the player can choose the tile for placement. Or else that all sea tiles bordering the city are assumed to have a harbour, though that would complicate where newly-built ships are placed.

Any prospect of this being addressed?

Edit:
This is in AoW III, BTW.

I don't know of any solution at the moment but this is worth posting on the AoW3 official forums over at Triumph. It's more likely that someone who can address it will see it there.