A tileset for the whole earth is VERY large. It is impossible to "have everything", so it is useful to understand how a tileset works.
An earth tileset consists of a "pyramid" of 256x256 pixel image tiles. Level 1 is the whole world in a single image, level 21 is the level of "spy on people on their rooftops" and consists of 1 million by 1 million images.
Of the 21 levels there are some key cutoffs:
The app will store for offline browsing all tiles visited. The whole world at level 17 (ignoring 70% ocean) would consist of 24TB of data. ie approx 400x 64GB iPads full. This is impossible to store, so active management of desired tiles is needed.
Level 1-8 is useful as a backdrop. For the whole world this is a maximum of 300MB of storage and 16384 tiles. This is roughly 6 "chunks" per sailing ground the size of Northland. The app will always keep levels 1-8 cached. At level 8 the currently cached size of the higher resolution tiles is shown and may be explicitly cleared out to recover space.
Levels 9-12 are blurred landscapes, useful for context but otherwise not useful for cruising.
Levels 13-17 are practically useful for cruising. At level 13 you just start to see breakers on large reefs, and at level 17 you see highly detailed anchorage information. For a single Level 8 tile the level 13 tiles only take up 20MB, but the full level 17 tiles would take up 4GB.
Levels 18-21 are unecessarily wasteful of storage. The app can display and store level 18-19, but level 20-21 are not displayable.
So in practise, the storage of areas of interest is quite practical. You could for example store information for 2-3 island groups at moderate level of detail, with smaller areas of interest at greater detail in 2-10GB of device storage.
Enjoy, I hope this is useful. But remember, this is not to be used for actual navigation as there is no guarantee whatsoever of any accuracy in the displayed GPS coordinates, or of the tile content imagery.
Marijn