Here's a response from my brother to your question:
"Yes, you can get nautical charts for your iPad and I suppose you could use them along with a paper chart for navigation on small private vessels. My question is why? There are so many inexpensive chart plotters out there that are more suited to operating in the harsh marine environment. Most will now interface with the vessel's radar and give you a picture with chart and radar data. You can get a good one for under a $1000.00 and an ok one for around $500.00. I personally would not take my iPad out on my boat unless I had a water/shock proof case. I would not use it aboard a commercial vessel for navigation."
Since I was an airdale in the Navy and never learned the correct nautical terms and all that, I can't give you his credentials, other than "his job involves boats" and he's got some licenses and certifications