INHO, for what is worth, which is not much.
None of these solutions are magic bullets that will guarantee being rescued. You need to do your homework to see what is best for where you are diving.
The lifeline is probably the quickest if your dive vessel is DSC/AIS equipped and is being monitored and you have programmed in the correct MMSI, or you are in an area where DSC/AIS is common and monitored. The limitation is the effective range. A boat with the aerial 30 foot high will have a range of around 6.7 miles while a 10 foot aerial will be around 3.9 miles. Find out the boats MMSI and any group MMSI and make sure it is programmed. Depending on where you are the lifeline may only send AIS and not DSC signals and may only send the signal to your boat for 30 minutes before sending a more general alert.see below
Here in Australia DSC is not mandatory for recreational vessels, but it is monitored by shore stations if you are in range
A PLB will get a response, your distress call is escalated through a local user terminal, mission control centre and then the rescue coordination centre (RCC) responsible in that region for arranging search operations.
If your beacon is
registered, the details are provided to the RCC in the country in which the beacon is both activated and registered. I keep my details updated when on a trip to the extent of saying diving with XXX dive op, and the ops telephone number.
It will take time for a local SAR operator to be notified, and for them to mount a SAR operation, how long is the great unknown.
The Garmin is dependant on someone at Garmin contacting the appropriate people to start a search, same as the PLB, Can the distress call have the ops contact details in it, that might speed things up?
The biggest SMB that you can keep vertical for a long time seems like a damned good idea, but remember a person whose eyes are 6 ft above the water will only see the top of a 6ft SMB out to a distance of about 4.2 miles
I do not own a lifeline, but I have read the manual. If you have programmed it according to the manual, then the following limitations will apply
Select “Device Mode”. Choose the region where you will be using your Nautilus GPS.
• Canada: Only AIS will be transmitted. DSC not permitted due to local regulations.
• Europe: Only AIS will be transmitted. DSC not permitted due to local regulations.
• USA: Full AIS alert will be transmitted. DSC alert will be sent first to your programmed ship MMSI. After 30 minutes, transmission will switch to your programmed group MMSI. Refer to steps 4 and 5 for details on MMSI programming.
• International: Full distress alerting with AIS and DSC.
If a ship MMSI has been programmed, DSC will first be sent to your own ship. After 30 mins., DSC will be sent to all ships.
So in Canada or Europe you will need to be in range of an AIS equipped station