Bill - I'm sure you've already thought of it: the bluetooth unit will need to be connected, usually via USB, to charge regularly.
If you have a serial
port available, these mushrooms are tiny and waterproof, not to mention half the price
: USGlobalsat Corporate
The main use for these are to provide an inexpensive NMEA 0183
GPS, that works very well for input to chartplotter
, DSC radio
, etc. (A USB adapter is available.) I use them below the deck
as well - I just don't want to chase cables outside.
If you really need USB then go with Rebel Heart's recommendation. Or any other GPS USB stick that has at least a SirfStar III chip in it. These newer products work just fine down below (often better than $300 "marine" products).