Shot, there is a difference between a regional deterrent and a global one.
Arguably the Israelis could mount an IRBM on a fishing boat and have a viable deterrent.
Given our desire for a global deterrent we may need a more robust system. Then you could counter that only Russia could mount a viable MAD with smaller players fielding sub-strategic threats, ie not blanketing the whole country. A ship borne system might therefore be sufficient.