At the risk of sounding like one of the people i regularly deal with…Are you sure you want a ‘stepper’ motor? Why not use a simple servo? I flood my servos with mineral oil (which is non-conductive) so they work outside the ROV. The water pressure won’t crush the servo since it’s filled with oil, and water also has no reason to leak in along the ‘spindle’.
For most of the bigger metal-gear ones, i’ve drilled a small hole in the housing and plumbed in some 1/8" aquarium hose, which goes to a resivoir, that i can push fresh mineral oil into after each mission, purging any water which may have leaked into any of my servos. After many hours of use, servos with resistive-strip feedback can become polluted by carbon dust, but by purging fresh mineral oil through them this issue is eliminated. The mineral oil Will leak Out around the spindle, under purging pressure.
I have (2) 9-degree-of-freedom manipulator arms which are actuated wholly by servos, not hydraulics. I find servos turning leadscrews to be plenty powerful, quick and precise.
If i needed more speed and power, i would buy a 20v Li-ion brushless DeWalt drill. After potting the stator in epoxy and replacing the bearings with ceramic ones (for saltwater immersion), you have a powerful brushless motor turning a planetary-gear-reduction gearbox which is very fast. Then turning leadscrew to make a linear actuator. The drill can be bought cheaply (without the battery or charger) on Amazon.
Going a bit further, i use an SSC32 servo controller to power all my servos and lights, controlled by the Raspberry Pi over I2C. I use a 32 channel ADC to get pot inputs into my topside R Pi. The only thing i use the Pixhawk for is thruster control.
I don’t have any pics of my setup cause that laptop locked up, but here is my original design for you to examine…
I mounted microcameras on the manipulator(s) wrists so i can more easily maneuver the gripper into position. Just watch how excruciatingly slooooww any of the professional ROV manipulators move…jerk…move…correct…turn…jerk, drop, and you’ll see…they may have megabucks, but they can’t design for drek.
I have an 8-camera security-system DVR in the ROV connected to a 4-port ethernet switch sending all 8 camera feeds up the tether through the FathomX interface.
Long reply but…i’m here to help.