Google Maps? Download maps around the route you expect to take & use them offline. You'll need a fairly large amount of storage though. You could probably get away with only downloading detailed maps around the islands you'll be visiting on the way as large expanses of ocean have no detail to display.
Here's a novel thought - acquire a paper chart (or print something suitable) and simply plot your GPS derived coordinates. Or use a combination of both - paper chart for the deep blue bits and the downloaded maps near land.