A navigation app turns your phone into a chartplotter, though they vary more than the marketing lets on. A few things genuinely matter when you pick one. Most of the rest is noise. These are the ones worth weighing.
Official charts
Charts are the foundation, so start there. In US waters you want official NOAA charts, the government's own data, with accurate soundings, marked hazards, and proper aids to navigation. Proprietary charts range from excellent to years out of date, and you can't always tell which you're getting. NOAA is the safe default.
Routing that respects depth
Drawing a straight line between two points is easy and often wrong. A useful app plans a route that keeps you in water deep enough for your draft and bends around shoals, rocks, and closed areas the way a navigator would. A safety score or a warning on the route is a real bonus.
Weather on the chart
Wind, waves, and rain radar drawn over the chart, instead of in a separate app you have to flip to, let you decide at a glance. Animated wind and a forecast you can scrub through are what let you time a trip around the weather instead of guessing.
Offline
Cell coverage vanishes a few miles out. The app has to let you download chart regions in advance so it keeps working with no signal. Below that bar, it's a harbor toy.
Easy to read underway
You'll glance at this thing in glare, spray, and chop. A clean screen with a night mode and chart detail you can dial up or down beats a spec sheet full of features you'll never open.
Price
Most serious apps run on a subscription or a trial. Take the trial and test it on your own waters before you pay. It's the only way to know how it handles the places you actually go.
So which one
Look for NOAA charts, depth-aware routing, live weather, offline downloads, and a display you can read at the helm. Pathr was built around that shortlist, and you can try it free for two months on iOS. See how it works, or start with free tide charts.