Hey David,
I'm sure Steve will add some good advice as well, but I have the 100mmL and LOVE it. Above water it has become my favorite lens of the moment, it is amazingly sharp, I would say even slightly sharper than the non-L version (which was already real sharp) and the IS is a huge advantage for hand held above water.
That said, underwater all the above applies but the IS is much less of a player and even Canon states that the IS when shooting Macro, close to 1:1 only offers maybe 2 stops of stabilization. I still keep the IS on underwater and have not really been able to tell if it helps any or not, certainly does not hurt in my opinion though.
I would say if your only concerned about it underwater and are looking to save $$$ the regular 100mm would be a beautiful lens, if you want a really stellar above water long lens as well, the 100mmL is very very sweet and worth every extra penny in my opinion!
Dear Steve,
How helpful will image stabilization be on the 100mm for a Canon T2i? The Canon EF-L IS 100mm vs the Canon 100mm EF-S?
Thank you,
David