From Wikipedia comes this: <<A hydraulic system under the control of the driver could swivel the propellers; the commander from a platform at the rear of the turret, where he could see over the skirt, could contribute by operating a large tiller.

Complete story about DD Shermans here: https://en.wikipedia.org/wiki/DD_tank
As mentioned earlier, the gunner could easily stand behind the turret and would be on the correct height to operate the cal50.
If you wish to put a figure behind the cal50, just create a small platform and put it on there. Not everything always needs to be "historical correct" .