Sigma 150mm f2.8

GREAT lens. Very difficult to use underwater. The ultra-sonic focusing is way too fast. This lens is beautifully sharp and when shot wide open you can get real buttery bokeh. Probably better as a land lens where you are not dealing with all the chaos of the underwater world but if you are up for a challenge it can be a very powerful tool underwater too. You just need patience. There is a dedicated 1.4 teleconverter that is available and it renders images with very little degradation.