With the low velocity comes much less accuracy on a heaving and pitching ship. A RC rifle will have to be a short range weapon to give a heavy punch during close in fights, unless it is used by anchored ships in shore bombardment.
It'll still be a fairly short-range weapon then, albeit a somewhat more accurate one. Because of that low velocity it'll also have a relatively long flight time and a pretty curved flightpath for the shell, which won't have much kinetic energy and will rely on its HE filling for effectiveness.
The main merit I can see for the idea is the lightness of the weapon, potentially allowing it to be shoe-horned onto much smaller hulls. As has been mentioned, it might find a home as a close-range weapon for destroyers and MGB/MTB designs.