I've got the Marine Imray Charts app which you can update with any of their charts, with an in app purchase. Although I seem to have the whole of the English Channel but can't find a receipt or remember specifically buying the charts, so maybe I got a free offer at sometime
Works well on both iPhone & iPad - but I don't regard either as being rib friendly - far too delicate especially the iPad with its large easily damaged screen - even if protected by some sort of waterproof case. Also neither screen is a patch on my Garmin chartplotter for visibility in bright sun. And any app that uses the screen & location services all the time, eats the battery life.
Yes you can 'tether' the the 3G/GSM signal from your iPhone if you have tethering activated on your mobile contract - works well but a further drain on battery. And of course its only economic if you use smallish amounts of data and certainly not for roaming 'overseas'.