It may not be the electric side of shift control valve. The transmission (reverse gear) is hydraulic. The shift control valve is hydraulic valve assembly that directs the flow and pressure of hydraulic oil to either fwd clutch pack or Reverse clutch pack or neither which would be neutral. The "normal" solenoid failure would cause the reverse gear to fail in neutral. This is not to say it is not a solenoid issue just unusual. The fact that there are no codes suggest the electrical side is not sending a failure.
It could be a mechanical issue, stuck valve, broken spring, or debris in the control valve. It could be clutch pack failure do to excessive heat which may have warped the clutch plate and the fwd clutch are staying compressed or engaged. A simple test is see if you can turn the shaft with the engine off. You will need a fabric strap wrench to do this. With the engine off there is no hydraulic pressure in the transmission so the the clutches should be relaxed and the shaft should turn.
Things to look for if a mechanical failure, burnt oil, filter with excessive wear particles, small particles visible to the eye. Using a 1/8" or 1/4" stainless or copper tube with a oil pump and a clean container. Put the tube in the fill/dip stick opening and work the tube to the bottom of the transmission suck a small amount of oil off the bottom of the transmission. Inspect this oil for excessive metallic and visual metal particles. ( there will always be some metallic and small metal fragments even in a serviceable transmission) You are looking for excessive. Your Mechanic should be able to distinguish between normal and excessive.
If the control valve or components of the valve have failed. I would recommend an oil change and filter. I would then send an oil sample to get an oil report as to the viscosity, water contamination, and levels of contamination of metal particles. If there is a failure to the reverse gear the rebuilder will be able to diagnose the failure and determine a cause of failure.
Hopefully it is a simple fix and you will be back on the water soon. I would recommend asking as many questions as possible to understand what the failure is to confirm the root cause is determined and the gear is fixed properly. Good luck.