Why a modern SSBN cannot casually launch from the surface

Can a modern ballistic-missile submarine surface-launch its missiles and then submerge safely?

Not as a routine option: modern systems are designed for submerged ejection, and a surface launch could fail or damage the submarine badly enough to prevent submergence.

Early systems that could launch from the surface differ from modern submarine-launched ballistic missiles designed around a submerged firing sequence. Water pressure, density, missile ejection, and ignition are all part of that design. Ballast systems can compensate for the departing missile's weight, but they do not replace the surrounding water or protect the deck and hull from an unsuitable launch. Even if a surfaced boat forced a missile away, the resulting damage could prevent it from submerging again.