This relationship (Depth = PSIG / .445 ) will give a 3% error if you dive your boat in fresh water.
In imperial units, you should use Depth = PSIG / .433 for fresh water and Depth = PSIG / .445 for saltwater. In my boat the user can touch the display to change from seawater to freshwater for this depth calculation. See Sean's correct derivation of these relationships.
Cliff