Garmin GPSmap 421s Marine GPS Review

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Space is almost always at a premium anywhere on a boat, and electronic devices can quickly clutter up a cabin. That’s why the Garmin GPSmap 421s is popular with boaters the world over. It integrates a GPS chart plotter with a depth sounder in one compact package, but it still has room for a big, bright screen that makes finding your way on the water easy and safe. Everything that’s installed in a marine environment has to be able to take a lot of abuse, and the GPSmap 421s is built with Garmin’s well-known attention to toughness and water resistance.

Chartplotters and sonar devices have to be judged on a more stringent basis than handheld units for hiking or turn-by-turn direction finders for the car. If your GPS device is off a little when you’re traveling on the highway, you’re not likely to start driving through the woods just because the unit told you the highway was fifty feet to the left of where it really is. But on a boat, especially at night, you need rock-solid satellite reception coupled with pinpoint accuracy if you’re going to be able to rely on a navigational unit to see you back to port. The Garmin GPSmap 421s offers great performance, and you can take advantage of high-resolution satellite imagery, auto-guidance functions, and 3-D views to keep your boat off the shoals and on course.

Garmin GPSmap 421s Marine GPS

garmin gpsmap 421 Marine GPS

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  • Compact marine chart plotter and depth sounder
  • Weighs 1.3 pounds
  • 5 inches x 5.7 inches x 2.7 inches thick
  • 4-inch full-color display
  • 240 x 320 pixel screen resolution
  • Waterproof to IPX-7 standard
  • Keeps track of 3,000 waypoints, 50 saved tracks, and 100 routes
  • 1,500 feet depth sound
  • Adjustable keel offset
  • Built-in alarms for fish, shallow water, deep water, and low battery levels
  • Included tilt and swivel mount
  • Comes with a protective front cover for when you’re anchored or in port


The Garmin GPSmap 421s comes complete with a worldwide marine basemap. Most users will want to upgrade to detailed charts for the particular area where they’re usually on the water. The Garmin is equipped with a very fast processor, which is important for a device that keeps track of not only your position, but also the depth of the water under your keel. Any lag in the information displayed on your screen could cost you a bent prop, or worse, so users will appreciate the 421s for its fast locking on satellites and quick refreshes.

Precise Installation and Calibration of the Transponder Is Important

The depth sounder has a transponder that needs to be accurately affixed to the boat in order to give usable readings. When a boat rolls, the angle of the transponder changes a great deal in relation to the bottom, so it’s important that the original reading of the bottom be calculated as accurately as possible in order to have an exact baseline reading to compare it to.

If you’re using the Garmin GPSmap 421s to poke around for fish to catch, you’ll appreciate the device’s ability to show a split zoom display that tracks targets while still showing the features on the bottom on the water. If you’re trolling or motoring to the next spot, you can set alarms based on fish size or water depth to alert you to the presence of a lunker or an obstruction.

An Empty Sky Helps, But Marine GPS Should Be Better Than On Land

In some ways, GPS navigation aids are much easier to design for watercraft than for land uses because there are so few obstructions when you’re out on the water. With so much clear sky available, finding and locking onto satellites should be easy. While that is certainly true, it’s when visibility is low because of fog or other bad weather that you rely on devices like the Garmin GPSmap to see you through. The 421s is as good as any compact GPS unit at finding and keeping satellite reception when you’re on the water and getting you back home safe.

The Garmin GPSmap 421s comes with a lot of minor useful features that you might be interested in from time to time when you’re on the water. It supports AIS ship position targeting, so you can avoid collisions. It also supports DSC technology that displays position data from VHF signals from radios on boats. The unit has tide tables, hunting and fishing calendars, and sun and moon information.

As options, you can purchase BlueChart g2 Vision cards that let your unit show 3-D map views and gives auto guidance information. If you already have Garmin sonar on your boat, the Garmin GPSmap 421s is compatible with it, and that might save you having to install another transponder.


  • Plotter acquires a fix quickly and accurately
  • Rugged case and waterproofing make it stand up to the tough marine environment without flinching
  • Internal antenna avoids cluttered cabins
  • Menus are easy to use and logically laid out
  • Screen is easy to read even in harsh outdoor light


  • Adding detailed local maps can get expensive

Final Verdict

Garmin GPSMAP 421s 4-Inch Waterproof Marine GPS and Chartplotter with Sounder

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Navigational aids like the Garmin GPSmap 421s really shouldn’t be considered optional in this day and age if you’re venturing out into open water, even if only occasionally. Relying on other electronic devices like smartphones for navigational guidance is a recipe for disaster on a boat where the marine environment can quickly cause a failure in any device that’s not built for it. Luckily for boaters, marine GPS technology like the Garmin GPSmap 421s is affordable and reliable, so the days of determining the depth of a channel by bending a propeller are long past. If you’ve been reluctant to go boating at night because of understandable fear of losing your way, the Garmin GPSmap 421s might be just the thing to give you peace of mind on the water, especially after you use it during the day for a while first to get used to it.

About 65 percent of purchasers rate the Garmin GPSmap 421s at four or five stars, and almost half give it the highest rating possible. Users grumble a bit about the cost of detailed maps, but acknowledge that even printed nautical maps can be pretty expensive. All in all, the Garmin GPSmap 421s is rated a best buy.