Hello David and welcome to the forums.
In this game, you cannot sail around the world in real time. But you can sail from San Fransisco to New York via a (short section) of ocean or oceans, for example.
You can transfer to another environment, ingame, during a mission, and not just one, but a whole series of transfers if you so wish.
So you could start in Rotterdam, Then Transfer to the NorthSea, then transfer to The Solent, Make a stop in Southampton, Head back out and continue trough the Atlantic Ocean, to arrive in New York Harbor. Or any other combination you might like.
These open ocean enviroments are (off the top of my head) 10 square miles. So big enough to make for very long missions if you link them together with ports. And honestly, who has time to go on realisticly long trips anyway.
You can't sail to the Moku'āina o Hawai'i in this game, but you can sail to the Phi Phi Islands and they are very pretty indeed aswell. Although these are not in the pacific.
But you can sail the Pacific Ocean though, well, again, part of it.
Multiplayer to this date, is just a gathering of players on a certain environment in Free Roam mode. You can sail and chat and do "pretend" jobs, in absence of any objectives or waypoints. But you can tow or push or otherwise interact with each other.
In the future, there will be more possibilities to host missions that we can play together and the likes.
I hope this gives a clear picture of what to expect. There's more of course, like the missios editor, with which you can realise all these things you might want to do. There's ingame downloading of user made, custom missions. A ranking system that progresses with your experience, and much more.
I hope this helps a bit!!
Regards,
Fred