David S, the Apple iPad comes in several flavors. In the past, it used a "30-pin Apple dock connector" to plug in for charging and USB connection to computers and such. The newer models use a smaller "Apple Lightning connector" which can be plugged in either right side up or upside down to accomplish the same. (Apple is currently selling only iPads with Lightning connector. The 30-pin dock connector is deprecated.)
An iPad with only wi-fi networking does not have a GPS chip built in - to get GPS functionality you'll need either a GPS receiver. The Bad Elf makes both a GPS device to plug into the Lightning connector, and a GPS device for the older 30-pin dock connector:
http://bad-elf.com/pages/products There are also GPS products that connect to the iPad using the Bluetooth wireless connection that is built in to all iPads. The Dual is an example:
http://gps.dualav.com/explore-by-product/xgps150a/ (Bluetooth is a short-range wireless connection standard often used for phone headsets, wireless speakers and the like.)
However, if you buy an iPad with cellular and wi-fi, the cellular chip inside the iPad contains a GPS chip. You do not need to enable the cellular service EVER in order to use the GPS functionality built in. You will, however, have to pick if you want your iPad to have a Verizon or AT&T compatible cellular chip, because VZ and ATT networks are not compatible with each other. Again, you DO NOT need to ACTIVATE a cellular data service on one of these iPads. Don't pay for a data service if you don't want it.
More info on the current iPad lineup is here:
http://www.apple.com/ipad/compare/
Many people feel the iPad is expensive, and they opt for a less expensive tablet that runs the Android operating system. I hear some folks have good results. I don't use Android devices, so I can't help. But what I can tell you is that you often get what you pay for…
I have been happy with my Apple iPhones and iPads through 4 different models now. I generally use my iPhone to provide GPS speed and direction on the boat. I would like to use an iPad Mini with cellular, but I honestly don't have the need for a chart plotter where I sail, so the iPhone showing me speed readouts is good enough. If I were to sail coastal areas like Barnegat Bay in NJ or the Upper Chesapeake, I would break down and buy a GPS adaptor for my iPad wi-fi only, or maybe a new iPad Mini...
If you choose an iPhone or iPad to help you with onboard navigation, I suggest the LifeProof line of cases:
http://www.lifeproof.com/en/ I am excited to use the LifeProof suction cup mount with my iPhone 5s and LifeProof case this summer. I'll be able to stick it right to the back side of the cabin, or maybe on the inside of the transom right under the tiller. But I think I'd rather keep it in front of me, in the direction I'm headed and looking most of the time...