I have the Ipad 2 with 3G and, therefore, built in gps. However, I didn't trust that the gps receiver was going to work well when down below, so I bought a bluetooth receiver as well. I had thought this was the only one that worked with the Ipad, at least at the time.
When I turn it on, it works perfectly. Automatically synchs with no effort whatsoever. It is small, charges with no effort and comes with a suction cup holder to mount somewhere. The downside is that it can't get wet, although could easily be put in a waterproof bag, such as designed for cell phones. It also has a swipe to turn it on and off, like the ipad's confirmation swipe, no actually buttons. It is easily turned on and drained when in a bag. I have a 12v dc plug in the cockpit and usually just leave it plugged in when using it.
Amazon.com: GNS 5870 MFI GPS Bt Receiver "WAAS" Enabled: GPS & Navigation
As it turns out, the built in receiver actually works just fine when below and I rarely use this.