The energy amount the rocket impacts with is dominated by its speed, not its mass. A small, fast-travelling projectile can make a hole in just about any kind of

The energy amount the rocket impacts with is dominated by its speed, not its mass. A small, fast-travelling projectile can make a hole in just about any kind of armour. The terminal velocity is quite low, but that's in part because of the way it is engineered — an ICBM would probably be able to put a hole through a ship deck with ease. While what you've said is true, in practice it isn't a problem. If the F9 first stage is out of control and going too fast, it will almost certainly miss the barge to begin with. The barge is small, the ocean is big, and the F9 has to maneuver very precisely to reach it. If it does reach the barge, it will likely be moving relatively slow, and thus is less likely to sink the barge, even it things go crazy at the last moment.