Hydrostatic Release Unit

https://www.matchausafety.com/products/hydrostatic-release-unit.html

A lifeboat hydrostatic release unit (HRU) is a safety device commonly used in marine and maritime applications, specifically with life-saving equipment like life rafts and EPIRBs (Emergency Position Indicating Radio Beacons). The primary purpose of an HRU is to automatically release life-saving equipment from a vessel when it sinks or is submerged, ensuring that the equipment floats to the surface and becomes accessible for use.