Hi
I'll add mine,
http://www.worldofwarcraft.com/
It's an oline role playing game. There is water and you can swim in it. There are a few quest to do, like gathering pearls. You die if you stay too long under water. Unfortunatly, it cost a small amount of money every month to be able to play it, but it's a great game that has a lot to offer. It's very popular, there are 6.5 million people playing it.
