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,
radar,
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).