There is a free app called OVITALMAP (it runs on several devices including the iPad) that we found very useful when close to any land. It uses
Google images and you need to zoom in to any area that you may be interested in while you have cell or
wifi connection. The app then saves those
images and you can use it while offline to navigate. You can turn on
tracking and it saves the tracks for you.
We have used this app while navigating in areas where the
charts are bad and are able to zoom in and see the passages thru very shallow
water. On several occasions we used the app in conjunction with
Inavx using
Navionics charts and the charts showed us ON LAND but ovitalmap images showed our track within the
gps accuracy of the ipad that we used. We use the
Inavx on the iPad for planning and general navigation but when we get close to anything we turn on ovitalmap and can see with better confidence. We have used the saved tracks to follow thru very narrow twisty paths thru
reefs in less than ideal conditions knowing that we had previously been safe in that area.
There is a small upgrade fee that allows more track saving space but I don't remember the coast.
Al