I think your son misunderstood. The Apple Watch Ultra doesn't have a satellite transmitter. Some models do have an LTE radio so you could use that to call for help if you're close to shore inside 4G/5G cellular coverage. SpaceX is supposed to start offering satellite LTE coverage through Starlink later this year but it's not available yet and unclear whether Apple Watch devices will be supported.
The Apple iPhone 14 and later models do have a Globalstar satellite transmitter for
emergency SOS signals. This only works in some parts of the world.
For offshore use, most divers would be better off with a dedicated PLB or Iridium satellite transmitter such as a Garmin inReach. These devices are more robust than a cell phone (a case is still needed for diving), have longer battery life, and better satellite coverage. Garmin dive computers can pair with an inReach Mini 2 via Bluetooth, allowing users to trigger an emergency message at the surface without removing the device from the case.