As with other types of radio controlled vehicles, hobby-grade RC submarines generally cost more and are better built than most toy subs. But there are similarities as well.
Almost all RC submarines have the ability to dive underwater (if they didn't we'd call them ships or boats), return to the surface, move forward and backward, and turn left and right. The main difference would be that hobby-grade submarines can generally go deeper and further away than RC submarine toys.
Beyond that, the differences in toy and hobby subs are pretty much the same differences between other toy and hobby RCs: Radio frequencies, cost, upgradability, and speed or range. Additionally, many hobby-grade submarines are typically built from kits or from scratch rather than purchased already put together.
