I actually saw a Science show on TV a number of years back where scientists had so saturated a tank of water with Oxygen, that they were able to keep a mouse swimming and breathing underwater without drowning. I have no idea whether or not this was bogus, I can't verify it however see the link below.
http://www.frca.co.uk/article.aspx?articleid=100112
I feel sorry for the mouse, he doesn't look too happy.