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I am buying one of these, but I'm a little bit unsure if it's really worth the extra money to get the L version with IS ? Has anyone compared these two? I don't have to much money to spend, but I don't want to buy the cheapest one if the other one is much much better 

I'm gonna use it underwater as well... And it's a bit hard using a tripod down there (  ), so maybe it will get easier getting a sharp picture with IS?! Havn't actually tried IS underwater yet

The IS is a big help getting a crisp image when it's you that is moving but not if it's the subject moving. But like you say tripods are hard to set up at depth so you will diminish some of the problems with IS. But if you are exposing mostly with fast flash at close ranges, IS may not be of much use. Your best friend will be the quick flash speeds and high sync speeds.

I have the non-IS version and it's an excellent lens. For the sort of macro and close up work and occasional use for portraits I do its about the only lens I have that I don't regret the absence of IS - it's almost always used by me either with studio lighting/macro flash or with available light on a tripod. I guess if you did bug chasing with it which is more hand held or available light portraiture it would be nice to have the IS. But the image quality on either should be excellent.

I did an update from the old version to the new and I think it was definitely worth it.
The difference in image quality isn't that great, but the IS is.
With the old non-IS one I would struggle to get sharp images at 1/125 seconds, with IS I get good results at 1/15 seconds, that's three stops longer shutter speed. With good luck I have managed to get sharp results at 1/8 seconds.
Remember that IS doesn't stop moving objects/people so it's not a solution for every problem. Remember also that IS doesn't work very well att macro/closeup distances.

While the image quality difference isn't great, I still notice more "punch" in colors and better performance at large apertures (larger than 4)

From what I've read the L maybe slightly less sharp at wide open. Probably a function of the IS.

Maybe so, but it doesn't have IS. And if you don't need IS (or macro) Canon's 135/2 is also a great piece of glass.