I built a city near the coast, a harbour on the coast, and a transport in the city. The transport appeared in the city but I was unable to get it to move.

I tried again on another spot, with the city apparently on the coast, but maybe it's in diagonal or something, in both cases I'm unable to get the transport out of the city.

How does one do this? Just put the ship into the sea, and maybe also board someone inside?

I managed by selecting both the ship and my sovereign to get the sovereign into water, and also once to apparently have him still in the city but the cloth map showed him in the waters...

So I'm quite confused. How do you use transports? I can build them but not put them to the sea.