I don't have the Sony 100m, but I do have and use the Sigma 70mm underwater.
With my a350 (and I think your a700) the crop adds 50% to the lens length so my 70mm becomes a 105mm equivalent (but still 1:1 magnification).
The 70(105) is a great lens for macro, but I have found myself limited with larger animals by the working distance - for example I cannot shoot a "whole" Giant Frogfish in Lembeh as the visibility prevents me from getting good results from a distance.
Provided you have clear water, or don't want to shoot larger animals the 100mm should work well.
Results like this are fairly straight forward (provided you can find the critter!)
