I think it is because the air show box over water is the equivalent of the airfield which by its nature is closed to people and vehicles. Also the box over water is usually a bit larger as it can stretch along the coast and also probably has a lower minimum altitude. I assume the air show box that is outside of the airfield will have a higher minimum than you can do over water.

 

Chris

 Why is it thought necessary to close the "airshow box"  off on safety grounds over water but not on the ground?

It's called "risk mitigation" Gerry. Because they can close off a designated area over the lake, but could not do so over land, which is precisely why the airshow is conducted over the lake be begin with.

 

Economics plays a role as well of course. The lower costs of insurance rates for having a closed and controlled environment more than offsets the expense of closing the area.

The AAIB Special Bulletin reports:
 

To date no abnormal indications have been identified.  Throughout the flight, the aircraft appeared to be responding to the pilot’s control inputs.
.

.

.
During the latter part of the impact sequence, both the pilot and his seat were thrown clear from the cockpit.  The pilot sustained serious injuries.  The investigation continues to determine if the pilot attempted to initiate ejection or if the canopy and pilot’s seat were liberated as a result of impact damage to the cockpit.

https://assets.digital.cabinet-office.gov.uk/media/55e993f5ed915d06a100002c/S3-2015_G-BXFI.pdf