As others said AF with the AF-ON technique is a standard today in macro photography. But manual focusing in addition to the AF is for some situation superior. I have the Subal 105 manual focus flat port which has a built-in crone for manual focusing on the Nikkor 105.
The manual method lets you focus exactly on a specific part of the object e.g. the eye of the critter without shifting the camera (focus and recompose) or selecting different focus points in the viewfinder. But this also has some disadvantages, e.g. you must use both hands (left hand to manual focus and right hand to release the shutter).
Manual focusing also requires a bright viewfinder of the camera (with a fast 2.8 lens as the 105) and preferably a housing viewfinder (45 degree viewfinder from Subal and others or GS180).
With AF and assuming your rig is well balanced (neutral buoyancy and also not forward or backward leaning) you can use one hand (the right hand) only for focusing pressing the AF-ON with your thumb and release the shutter with your finger while holding on to something (but not a coral!) and thus stabilizing your position with the left hand. I also like to use one of these "Lembeh sticks" for stabilizing.
- scalesr1 likes this