Hi Mark,
*Welcome to the community*
I'm no expert on multiplayer in SS08 but...
You can set up your own server.
You can decide which vessels you want on it.
You can decide which environment you want.
There are no objectives but there are some very successful servers set up, Irish Jacks is one and here is a link to a topic he started recently about what they are doing in the run up to the release of SSex.
http://80.95.161.114/shipsim/forum/index.php/topic,19441.0.html
Tore, (TJK), has a couple of servers running I think, (he has been busy lately so I don't know if they are still there).
Have a look around in 'Multiplayer' on the forum.
Regards, Dave