Strange Location re Trade Routes

Playing on a small continents map and my capita (Berlin) starts out on a small inland lake. The lake has nine water tiles- no fish.

I build a city on the one land tile on the end of the lake that is stopping connecting the lake directly to the sea.

Now I find that I can send sea trade routes to Berlin- as can the AI- yet I can't build vessels in Berlin nor can I sail trade routes from Berlin. The trade routes go through the city on the coast (Hamburg).

The situation is more amusing than anything and one I'll probably never see again.

Yea, it's funny, although not really inexplicable.

The game decides whether to allow you to build naval units based on the size of the adjacent water body (set to 10+), and is defined somewhere in the XML for (I think) each and every unit.... at least that's how it worked in Civ 4, which I don't think Firaxis changed in Civ 5. So theoretically you can enable work boats to be built on lake towns just by lowering that number from 9 to 1.

Trade routes are calculated using on the game's regular pathing system, which treats DOMAIN_SEA as "passable" for all friendly cities and water tiles, so that is why you can create trade routes like that.

If playing a mod with Global Passable Improvements implemented, this would also apply to forts/citadels/whatever.

Can a Mod please correct my horrible spelling in the title when you have time?

Done.