A water reservoir includes a water reservoir base configured to hold a volume of water to be used for humidification of pressurized breathable air, a water reservoir lid connected to the water reservoir base, and a compressible resilient portion configured to seal between the water reservoir base and the water reservoir lid. The base and the lid are movable relative to one another when connected whilst maintaining sealing therebetween due to the compressible resilient portion. A retainer is configured to secure the water reservoir to a water reservoir dock, and has a protrusion or recess to releasably engage one another when the water reservoir is received in the dock, with a reaction force to compression of the compressible resilient portion urging the protrusion and the recess into engagement with one another.