Except that she'll get tired in 15 minutes, can't come up for air, and can't get herself unbound. If she manages to get to a beach, she'll be raped relentlessly, etc etc.
Just poke a hole in the back of her neck, then she can go up, store air, and stay down there a good 20 minutes before she bleeds to death or gets eaten by a shark.
All she needs is a scuba mask and air tank etc. and she will be fine.
Always thought about this when I was younger (except leaving the arms free, mermaid style). The whole possible drowning thing always made me decide against it though.
