Just thought I'd share... For over a month, i've been using an iPad in place of the laptop I used to use on the boat. It's exceeded all of my expectations. I have the 3G version, so I'm connected regardless of whether I'm near wi-fi.
The new navionics HD version is great.. It's more readable outside than my laptop.
I have the same set up: iPad 3G w/ navionics HD. I agree that it works great. The iPad 3G come with built in gps whereas the wifi only iPad does not. I use mine without a 3G AT&T data subscription.
One caution, my iPad shut downs with high temperature warnings if out in the sun too long. I have to keep it in the shade or in the cabin. Similar problems have been reported by others on macsailing.net.
That said, I just used the iPad on a 70 NM round trip on the Gulf ICW. It worked wonderfully. But on another note, the tide info from navionics HD does not agree with NOAA tide prediction (or most other tide tools). I reported the bug, and navionics said they are already working on an update.
But I do have to agree, I would drown before giving up my iPad....