I just saw something pretty interesting in a West Marine flyer yesterday-
Turn your smartphone into a SPOT transponder. Basically, you buy a SPOT antenna, jack it into your smartphone's USB port and install the SPOT app on your smartphone. I like this, because I like combining gadgets instead of having piles of electronics.
The main concern is that smartphones normally aren't ruggedized or waterproof, so if you're going to use your smartphone or tablet as a communications and navigation tool, you need to protect it well.
I've found my Droid X to be pretty tough, but I would still isolate it belowdecks, in a dry-box.
S/V Old Shoes
1973 Pearson 30 #255