i use group mode with AF-C only for wide angle when action is involved, e.g. shark shots. IMO it doesn't really work for macro, best is single servo - single focus point and focus field moved to the "sides*. Focus on the eye and release. Others tried also with AFC and 3D-mode, but that did not work for me either; the 3D selected focus point was not stable / remaining enough when moving the frame for recomposing.
If I understand you right you want to increase the size of the focus area, i.e. to make the nose and the eye in focus. Imagine a nudy branch diagonal in the frame from top right to bottom left with its head towards the lens. In this example a larger focus area by a group mode would be contradictory since the 4 group mode focus points might randomly select the eye or the nose of the nudi (probably where contrast is higher) and the one or the other might not be in the same focus plane (depending on DoF). Here you want to focus on the eye and be happy when nose is still in focus too. Alternatively instead of focusing on the eye you could use the good old 1/3 rule, namely get the focus point on the first third of the image (1/3 of total distance from close to far) and hope that both nose and eye are in focus.
The user manual says that group mode is more for situations when camera is not able to focus in other modes in order to avoid accidentally choosing the background instead of the subject as such.
But best way is to try the different mode when shooting underwater and make your own experiences...