I went to Rhapsody for a few minutes today. I have a Garmin 546s Marine GPS. The system software on the unit was version 3.10 and the most recent version is 4.20. I found that the GPS had software from 9 versions ago. I used an SD card from my camera and used it to update it. It took a few times and took my laptop and used it to load the SD card a couple times and repeating until it worked.
I also recently bought some maps for the GPS. I bought Garmin Vision lake maps for Southeast US from GPScity.com that seems to have better prices than others I found. It seemed to work well with the GPS. It also has some basic maps for land. I'm looking forward to getting Rhapsody in the water to test it out.