Nothing beats an Otter case for protection and durability, but as pointed out they are bulky. For a waterproof case, the dry case is also very good, but I know that my friends in Iraq and Afghan, just use gallon freezer ziploc bags.
There are a couple manufacturers of good mounts, that are on arms, or just have clips. Sorry, i dont have any names. You might want to google for iphone mounts, as you may get more hits, and then see if any of those have iPad mounts too.
Be warned that the GPS chips built into the iDevices are not that accurate. If close enough is good enough they are an excellent alternative to dedicated GPS devices. If you want/need more accuracy, then you need a Garmin, or like, or an external GPS receiver, like the one from BadElf.
I use my iPad as my primary GPS for Coastal sailing on the Gulf Coast of FL. Since my boat is small enough, i just put it on the cabin table, which I can see easily from the cockpit. Although I am thinking about mounting it to a swinging arm at the companionway.
