Here's some additional information regarding Navionics which you may or may not know.

The iPhone and iPad apps have the same level of data - the difference is that the iPad app is optimized for the larger screen with higher resolution which makes it easier to use as a planning tool on the iPad - much like a full chartplotter. I have Navionics installed on both my iPhone 6s and my iPad Air - which has required me to pay the roughly $20 for the iPhone version and approx. $55 for the iPad version, as I recall. The iPhone version will work pretty well on an iPad and doesn't require a 2nd purchase, however its not optimized for the iPad.

The iPad version - if you have a GPS enabled iPad - is really a good backup to a boat's fully functioning chartplotter. Frankly, if I were just banging around in the BVIs, I wouldn't NEED to use either, however, being ashore with an iPhone and having the app might be convenient around a bar/dinner table for some casual conversation and planning. So, its very inexpensive and convenient with great purpose and value.

If you are planning longer trips, then I think anyone would be wise to have Navionics or some alternative installed on a somewhat dedicated iPad as a backup to (1) chartplotter, AND (2) paper charts. That's my purpose. That is particularly true if you only have a single chartplotter on the boat at the helm (most charter boats) and want to have an electronic chartplotter elsewhere (ie: salon; nav station).

And, having the higher resolution and screen size compatibility of Navionics iPad app is nice and, in my opinion, well worth the extra price for the app. I have full US, and Caribbean charts on both iPhone and iPad version.