PIAA Rules concerning water depth
A clarification of PIAA rule 2 section 7 (Pool specifications- equipment for swimming events)
Under PIAA rules :
If the depth of the water at any pool is questionable the starter/referee should measure the water depth in each lane and if any lane was found to have water less than 42" deep then the referee/starter should rule that all swimmers must start in the water. When the water depth is less than 42" (3'6") swimmers MUST start in the water.
If the water depth is between 42" and 48" then swimmers are allowed to start from blocks no more than 18" above the water's surface or from the deck.
Above 48" water depth 30" blocks may be used.
A swimmer is ALWAYS allowed to start in the water if he or she wishes as long as the correct type of start is used for the event being swum (facing forward when a forward start is required, back when a back start is required).
However, if the water depth is less than 42" in ANY lane then all swimmers MUST start in the water. This is a fairness as well as a safety issue.
If the water depth is 42" or more in all lanes then it is up to the coach/swimmer to decide whether to start in the water or from the deck. Pool deck marking are not necessarily accurate and are usually conservative.
The same reasoning applies to blocks. If any one block is broken/too loose/unsafe then all swimmers must start from the deck or in the water.
Thank you to John Vernon for the clarification