Airlocks are actually rather dangerous. Things can go wrong very quickly. An accidental rapid/unplanned decompression, even only across a few feet of pressure, can do real damage. Then there are the issues building the doors, which hold enough force that should they pop open they could kill. The biggest advantage of the moonpool approach is having the entire complex at the same pressure, with the pool acting as a passive safety valve against wild pressure fluctuations.
Yeah, pressure differentials are no joke. Look at the Byford Dolhin incident [1]:
> Hellevik, being exposed to the highest pressure gradient and in the process of moving to secure the inner door, was forced through the crescent-shaped opening measuring 60 centimetres (24 in) long created by the jammed interior trunk door. With the escaping air and pressure, it included bisection of his thoracoabdominal cavity, which resulted in expulsion of all of the internal organs of his chest and abdomen, except the trachea and a section of small intestine, and of the thoracic spine. These were projected some distance, one section being found 10 metres (30 ft) vertically above the exterior pressure door.
> The blood of the three divers left intact inside the chambers likely boiled instantly, stopping their circulation
