Hi Mik,
To add to what Aad said: In free roam there is a button to get the environment selection globe without the need to first go outside the current environment. You can then select another environment like the Pacific Ocean, and from there to another port.
However, you are restricted to just those environments supplied: New York, San Francisco, Hamburg, Rotterdam, Marseilles, The Solent (Southampton, Isle of Wight, etc.), Padstow in Cornwall, Phi Phi islands in the Gulf of Thailand, and a few sections of ocean.
The situation will be the same in Extremes, but with the addition of more environments. Those confirmed include Sydney, Bora Bora, and a part of Antarctica.
If you are considering getting SS 2008, I recommend getting the Collector’s Edition. It includes New Horizons and installs both in a single operation from the DVD. This avoids the need for a bunch of separate updates, which not everyone gets right.
Regards,
Marty