A few tidbits on this topic:
- AIS is not mandatory for ships less than 300 tons dw. Neither for military ships, I believe. So don't expect AIS from every rescue ship.
- An AIS transponder (i.e. a transmitter which makes your ship visible to others) identifies the ship using its MMSI number, same as its phone number on VHF DSC.
- IMO number is meant for larger ships (over 300 tdw). I copied this list from Wikipedia:
The IMO scheme does not however apply to:
- Vessels solely engaged in fishing
- Ships without mechanical means of propulsion
- Pleasure yachts
- Ships engaged on special service (e.g. lightships, SAR vessels)
- Hopper barges
- Hydrofoils, air cushion vehicles
- Floating docks and structures classified in a similar manner
- Ships of war and troopships
Wooden ships[1]
Note that SAR vessels don't need an IMO number.
As said, AIS main use is collision warning (much as TCAS, but closer to ADS-B) while the tracking function is an extra goodie.
I've had an AIS receiver in my sailing boat for a number of years and now have an AIS transponder (an approx $400 investment). Very good piece of safety equipment.