Actually there will not be any signifcant difference between these three models, since I am pretty sure all of them support the following features (even though at different price levels of course)
Hear-Rate Measurement – This is probably the most important and also the most basic feature any sports watch has. What might be different though is the way they measure the pulse. The majority of sports watch suppliers need you to wear an extra belt around your chest, which monitors your heartbeat. Some are able to detect your hearbeat at the wrist itself. However I discourage from using a watch like this as it can be a bit annoying to have the sportswatch around your wrist whilst rowing and they are usually less accurate.
GPS – Not every sports watch has this, however it is a feature that is worth investing into. It is harder to track distances on the water, so GPS is a good help measuring your total distance. Plus some watches also track what time you need for a certain distance. This way you can see whether you keep up the same pace or not.
Synchronizing of training data – This might actually just be important if you want to row at a somewhat competitive level and want to track your training. Rowing is a sport with a big focus on endurance and cardiac training. You will have different units at different intensities. An easy synchronization of data (e.g. duration, distance, heartrate average, heartrate peak) can increase the effectiveness of your training in the long run, because you will see to what kind of stimulus your body reacts the best.
Stroke Rate Meter – The measurement of the stroke rate is usually not implemented in a sports (wrist)watch. The stroke rate is an important parameter in rowing, however. You can use it as an indicator of how effective you are rowing and you can control your boat and yourself better during exercise. Yet, a stroke rate meter is not a crucial necessity (in the beginning).
I personally used a Polar (just with heart rate monitoring though) and really liked it. The battery lasted almost ten years before I had to switch it for the first time.
They are all good brands, so thankfully it’s hard to go wrong. I personally would give the edge to Polar, since they have the H7 Bluetooth heart rate belt. It’s compatible with all of the Concept2 monitors (with possibly some extra equipment):
Heart Rate Equipment
And it will also work with the NK Speedcoach GPS with heart rate:
The latter is somewhat irrelevant if you plan on purchasing a GPS watch. However the NK is a useful tool to have because it gives you splits and stroke rate all in one package.
Definitely go for one that has a chest strap for heart rate monitoring. I’ve read many reviews complaining wrist-only meters not giving accurate heart rates due to wrist/forearm movement during rowing.
Ideally you want a tool that gives you feedback on speed, distance and heart rate, and is waterproof.
I have personally used a Suunto Ambit for 3 years now, unfortunately I have no experience of other products. I have been very happy with my choice, the watch measures distance, speed and heart rate. I’ve also downloaded an „app“ that shows my speed as a /500m split. It’s tough make and has absorbed many falls and strikes to it and it’s water proof up to 30m. All the data can be uploaded to an online training log called Movescount, I use it as my training diary. The battery lasts around a week and can be recharged with a cable to plugs to a usb-port. Same cable is used to upload data to Movescount. The watch’s memory stores around 50 previous moves and it deletes oldest ones from the watch automatically.
It’s the Garmin Fenix 3. They have a rowing and indoor rowing feature. This may be the only sports watch with a rowing function specifically built in. The watch sensors (accelerometer) will calculate your stroke rate during exercise. I have the Fenix 3 HR. It was very accurate on the erg, matching the machine exactly.
I use a Polar HR monitor with a chest strap. I have a cheap one without GPS, because in the boat I use an NK SpeedCoach to track SR, meters and split times. The GPS SpeedCoaches are nice because they work in any boat, even without an impeller. I even use it when I cox as a „heads up display“ to get rate and splits, and just use the cox box for amplification