I can only speak for the iPhone and iPad. They have real GPS receivers and do not need cell reception to work. Without a cell signal to already know what part of the planet they are on, they will just take a little longer to initially lock onto the appropriate satellites. Once locked, it makes no difference.
With an iPad, you only get the GPS receiver, if you buy the version that has the 3G cell receiver. You do not need to activate 3G service for the GPS receiver to work, they just bundle the add-ons together.
I do not use either for primary navigation, but I do check them often and have used my iPad as a backup to a failed chartplotter. I cross check all navigation. Nothing is true, until two independent sources say its true. Typically, I use an electronic means as primary and my eyeballs as secondary. If I'm in familiar waters, I'm checking that land or nav aids are where the chartplotter says they are. If in unfamiliar waters, I am cross checking to a printed chartbook.
Here's a little secret. When I go below to use the head, make something in the galley, or whatever and my wife has the helm, I often pop on Charts and Tides on my iPhone and keep an eye on where we're going. Don't tell her.