Water activated ones work like auto lifjackets , 3 seconds or so under water and they activate
I found this
An EPIRB can be activated either manually or automatically when the EPIRB comes into contact with water.
Manual activation brackets will cover the water sensors, preventing them from activating the EPIRB if, for example, a wave breaks over the boat, and keeping it in a handy location should you need to access it quickly.
Auto Float-free housings, automatically deploy the EPIRB when submerged to a depth lower than 2 - 4 metres in the sea. They work by means of a Hydrostatic Release Unit which cuts the EPIRB free from its housing, causing it to activate. The EPIRB can also be removed from the auto housing to be activated manually. EPIRB PLB