This is a small rectangular section of an in-ground pool recessed into the top of the pool wall. This recess forms a small bench or platform. It is refered to a "swimout" in the commercial pool industry.
The "swimout" became the "deathbox" as skaters (i.e. the z-boys
) began to ride the vertical walls of pools in much the same way as the face of a wave can be surfed. This recess offered the skater a excellent opportunity to leave the surface of the pool altogether, catch some air, and then drop back into the bowl of the pool. If re-entry was not done correctly, things could go horribly wrong, and thus the name "deathbox".