A payload delivery unit for protecting and delivering a payload submerged in a submersion medium comprises a container including a pressure resistant shell and a resilient seal device. The shell defines a containment chamber and includes first and second shell members having opposed first and second sealing faces, respectively. The seal device engages and is interposed between the first and second sealing faces. The container is configured and constructed such that: when the submersion medium applies an exterior pressure to the first and second shell members such that a shell pressure differential, defined as the exterior pressure less an interior pressure of the containment chamber, exceeds a prescribed pressure, the first and second shell members compressively load and deform the seal device to effect a seal between the first and second shell members that prevents ingress of the submersion medium into the containment chamber; and when the shell pressure differential is less than the prescribed pressure, the seal device elastically rebounds to separate the first and second shell members to permit ingress of the submersion medium into the containment chamber.