Skildpadde can be used for both the land ones and the water ones (like turtle in American English). This means it can be translated either as tortoise or turtle.
Havskildpadde is more specific. It refers only to the water ones (hav means sea), so should always be translated as turtle. Landskildpadde refers only to the land ones, so can also be translated as either tortoise or turtle.
Does København has something to do with the sea? (it sounds a stupid question but in my mind it makes sense)