I disagree with Moses's strategy of using the 12-50 in Macro Port 65, which doesn't allow use of the lens' built in macro mode. This is a really compelling advantage, depicted here:
SMC_on_12-50.jpg 70.37KB 17 downloads
It probably isn't the highest quality lens available, but I don't know of any lens on any other format that gives the underwater photographer this range from almost wide angle to almost macro, then with the flip of a switch pretty serious macro. For someone looking for flexibility on a dive this is hard to beat.