Most propane tanks are constructed from seamless or welded steel. The design is a cylinder with a dome on either or both ends. The most common are 100psi or 500psi (the latter for transportation). At 500psi, propane is liquid and incompressible further. Assuming your tank is rated 500psi, that's for internal filling only. It is not RATED for external applications, so it does not mean you can apply 500 lbs per square inch externally, without failure. The design maximizes resistance to deformation when the pressure is applied internally; in many cases, a propane tank will deform when a crushing force is applied of substantially less than 500psi. If the tank is empty, deformation can occur at a lower level. If full, the liquid propane will provide resistance. The question is complex and application specific, including presence of other elements like fire or torque, and requires more data to give an approximate resistance.
Define the depth for which you wish to calculate the pressure. Determine whether the water is salt water or is it fresh water. Then multiply the depth in feet by 0.432 to calculate the underwater pressure in fresh water, in pounds per square inch (psi). If the depth of fresh water is 100 feet, then: 0.432 x 100 = 43.2 pounds per square inch. You can get more information on http://www.ehow.com/how_6397736_calculate-underwater-pressure.html#ixzz1hAd1n18N.