Hmm interesting. Couple counterpoints (just debating for the sake of synthesizing all this info) I'd love your thoughts on:
1. Should be pretty easy to put the InReach Messenger into a generic dive case, since it's smaller, so seems like that's not a decision maker, right?
Size is one consideration. If you have a Garmin Descent, there may be another. The Mini2 is compatible withe the Descent, so it can be operated from the watch inside the case. So, you could have it clipped to your shoulder strap, and operate from your wrist.
Garmin.com doesn’t list the Messenger as being compatible.
2. If you're worrying about being lost at sea, wouldn't you want a PLB or EPIRB instead of a satellite messenger? (Stronger signal, radio capabilities so nearby boats can respond, etc)? Basically something like a Nautilus Lifeline or better?
You are talking about a couple different devices here. A PLB/EPIRB and a Nautilus Lifeline.
The InReach is probably most comparable to the PLB/EPIRB as both use satellite COMMS. The PLB/EPIRB broadcasts to a network that is monitored by Coast Guard or equivalent SAR group. The InReach uses a network that is monitored by a private entity. Distress signals would need to be passed on to Emergency services. I think this was likely more of a concern before Garmin acquired InReach. The InReach allows users to send messages (and receive) not just for emergencies. ACR does offer sending messages to up to 5 contacts for an annual subscription, but it’s not nearly as advanced as the InReach from a messaging point of view. As I understand it, you need to preconfigure the message and use the self test functionality to send it.
Some of the newer EPIRBs do allow for communicating with nearby boats via AIS, but I’m not sure that functionality has made it to PLBs just yet. AIS is more common on commercial vs. recreational boats. So, depending on the area you could be reaching many boats, or just a few. The older Lifelines used VHF and DSC which is on most boats, though still not a perfect solution as many recreational boats don’t have DSC, or if they do, don’t have it networked to their plotter.
Currently no one solution is perfect. However, any one of them is better than nothing.