Abstract: A backpack for overland and underwater use having a central compartment for a first tightly-sealable sack adapted to contain the user's personal belongings, a rear compartment having a second tightly-sealable sack containing a sheet of foam material usable as a mattress, and a front compartment containing an inflatable bladder adapted to provide the backpack with good floating characteristics such that the backpack can be used as an element for logistic support and/or as a safety element for a skin-diver. The bottom region of the various compartments is provided with drainage for water. The backpack has straps which are capable of attachment in two ways, one for walking, the other for use in water. A line is provided for tethering the backpack to the user when underwater. The backpack is floatable when the bladder is inflated and a signal flag and flashlamp can be displayed.