Building a ship in a town that borders a sea AND a lake?

[RatofLuke]

This may be a stupid question, but then I don't want to build a ship only to discover it's trapped forever in a lake. But if I were to build a ship in a town bordering a lake and sea . . . how does that work? When I undock, where would the boat be?

[SumBum]

When you undock from a bridge location like that, it should ask you if you want to undock to the sea or lake. Just like locations that give you the choice of which direction to undock toward. And I think if you build a ship that cannot sail on a lake, that option won't appear at all.

[SekoETC]

If the location borders a sea and a lake then you can use it to pass between the two, provided that your vessel can travel on both sea and lake.

[Talapus]

Seko and SumBum have it right. An important thing to remember is that there are some boats that can only be sailed on lakes (ferry, rowing boat, canoe). Check the wiki, and build a boat that can sail on the sea. Then when you undock, just choose a direction that will put you out at sea.