I don't believe there is any one device that will do as you ask, that is auto-activate, set off a local alarm
and transmit to all other boats in the area. What there are are: Personal Locator Beacons (PLBs), AIS
Beacons and some specialty devices.
The PLB transmits a location signal to the Search & Rescue satellite
system which then alerts rescue
forces. To my knowledge there is no current
PLB that auto-activates when someone goes overboard
. Nor does the PLB automatically signal nearby vessels.
Contrast that with the AIS
beacon which when activated which transmits a radio
signal that can be received by vessels in range. Some can be activated by the inflation of a lifejacket if set up that way, I am not sure if any auto-activate upon submersion, there may be, just didn't find one. Note the AIS beacon will not alert the SARSAT system- thus if there's no one close enough to receive the signal (not every boat
can receive AIS) no one will hear.
I don't believe there is a combined PLB/AIS beacon approved in the US yet.
There are also some signalling devices that do activate automatically and trigger an alarm (via smartphone app). No endorsement- the one I found is called "crew watcher." It's an interesting device which seems to work
on the idea that if the receiver moves a certain distance from your smartphone (as in going overboard) it alarms. But other then providing an immediate signal back to you it seems of limited use in any kind of search situation.
Here's an interesting article: Here is an article that talks about this: https://www.soundingsonline.com/voic...-ais-satellite