a narco sub could easily be 50x the weight, needs to move significantly faster (RCTF boat is going like 5mph tops), and is fighting against extremely strong currents and waves (RCTF is on extremely still calm water).
But I'm thinking of a sub that is mostly just a solar panel. You don't need to be able to put a person in it? Don't you get a few hundred watts from a square meter?
It's the same reason it makes no sense to put solar panels on a car: the solar panels would add minutes of driving daily.
You would need massive area of solar panels to power a sub which is obviously not workable if you want to be stealthy.