I've been using the Navionics app, with good results. The GPS / maps are very accurate, little bit of a lull, but not bad. Came across a good description of it here:
https://citimarinestore.com/citiguide/best-boating-apps-for-boaters-anglers/
Of course, they're only as good as your cell signal as well. Getting a cell signal booster helps (not just for apps, but all around cell use of course).
Some of the apps for boating, GPS and otherwise, can indeed be battery hogs, as Bruceb mentioned. Wouldn't ever count on them as my main GPS, but helpful as a supplementary tool.
