It uses the Iridium constellation of satellites and seems quite reliable and works pole to pole. I had a small issue getting the account activated (many have, if you believe the chatter) but no big deal. It is a pocketable device that has a small screen and minimum user interface. It is "submersible to 3'" I think. It requires a paid account and this is not dirt cheap: the one I opted for is $25/month. This includes unlimited tracking, 40 SMS messages ($0.50 per additional), unlimited "preset" messages, and of course the SOS capability. The tracking is pretty good, giving your speed and altitude too (important in an airplane). Claimed internal battery life is 4 days with 10 minute track breadcrumbs. It will bluetooth connect to your smartphone, allowing easier typing of messages, etc. The preset messages aren't really a message, you must define them ahead of time and also the recipient list, might be something like "we are OK doing well" just as reassurance.

I would not call it a convenient messaging device, nor cheap. But compared to the alternatives (SPOT, sat phone, EPRIB) it had several advantages: SPOT is one way only, no way to communicate your current situation for example, no way to receive anything. An EPIRB is an all or nothing device. Sat phones are expensive to purchase and operate, are not waterproof, and have very limited battery life so their functionality in an emergency might be compromised. The inReach lets you summon help, but also put some color on the message - "we are dying, we are about to die, we are feeling really bad, boat is dismasted but OK", etc. Also lets others get a message to you: "we heard you and are on the way", whatever. It does that in a pocketable, waterproof package with reasonable battery life.

You can see my track when I was playing with it at share.delorme.com/JonFitch. Mostly you will see me driving to the Rose Bowl to watch Stanford get whipped on Jan 1. Go Michigan! (I learned that really pisses them off...). That map share facility comes with the tracking, lets people know where you are for better or worse. You can password protect the page if you like.

Had the Alpha 42 had one on board, they might have left it on, and been able to salvage more easily. You can do that unlike an EPRIB, which the authorities are going to want shut off when you get off.