To be a "tiger underwater" means to be really, really angry (or "super-pissed"). Taken from the actual event of a tiger being underwater. Upon searching "tigers underwater" on Google images, you will find that every single tiger underwater is abnormally upset with everything around it.
Ways to use it include:
"Dude I'm a tiger underwater right now"
"Yeah, he's being a tiger underwater"
"Dude, quit being a tiger underwater".
"You know, If I was just playing GTA4 because I was bored and I got the Red Ring of Death, I would have been pissed. However, I got the Red Ring when I was about to play a game I've been waiting months to play, so you could say I'm a tiger underwater."