A reader who has extensive knowledge on this subject has submitted a pre-publication excerpt from a research paper authored by Joel M Stager and Andrew Cornett that will appear in the Journal of Aquatic Research and Education.
Regarding shallow water mandates:
"...There are, however, two separate desirable outcomes of these water depth mandates:
(1) Having enough water depth to eliminate the chance of a swimmer hitting the bottom and...
(2) Lowering the chances of a swimmer being injured should contact with the bottom occur.
The second outcome seems to be the more achievable and certainly more realistic. With this in mind, we draw several conclusions.
The swimmers at the greatest risk for experiencing a catastrophic injury during the execution of a competitive swim start are the physically mature swimmers with limited practice and start experience. We make this conclusion based on two major findings. First, older, taller, and more massive swimmers have been repeatedly observed performing starts with greater maximum head depth and head velocity (Cornett et al., 2010, 2011). At the same time, swimmers lacking in competitive experience have been shown to be inconsistent in terms of controlling or modifying start depth (White et al., 2011; Cornett et al., 2012). This combination of deep, fast starts and a lack of control can have catastrophic consequences. Surprisingly, the younger, novice swimmers, in general, seem to be at a lower risk in this regard. The young, novice swimmers simply do not attain head depths and velocities great enough to place them at the same level of risk as equally inexperienced but physically mature swimmers. This is certainly not to say, however, that they are risk-free.
Next, we conclude, similar to previous authors on this topic, that the empirical evidence suggests that the current minimum depth of 1.22 m needs further careful consideration. It appears that there is very little margin for error at this depth, particularly for the older, physically mature swimmers due to their depths and velocities. A significant number of swimmers closely approached the pool bottom at this depth (Cornett et al., 2010) such that the potential for contact appears unacceptably high. Unfortunately, data from competitions held in 1.52 m (5 ft) are not yet available making firm recommendations on “how deep is deep enough?” difficult. "
It is certainly possible to require deeper minimum pool water depths than what is called for by current regulations. Doing so would likely help to reduce the risk of injury due to pool bottom collisions during the swim start. The studies presented in this review lead directly to this conclusion. However, while the risk can be minimized, increasing minimum water depth within the range of feasible water depths cannot eliminate it.
Our study of ‘worst-case scenario’ swim starts demonstrated that the potential for catastrophic head and neck injury existed at water depths as deep as 2.5 m (Stager et al., 2013). Thus, changes in minimum water depth requirements are less a matter of eliminating the possibility of contact and more about learning how to reduce the incidence of contact and severity of injury through coach and swimmer training and education. ..."