You finish a rowing interval. The machine has logged the metres, but your watch's heart-rate graph contains a blank stretch.
Before using that record to judge the session, check how much of it was actually measured. A gap cannot tell you whether those minutes were easy or hard.
Wrist optical sensors use light to detect changes in blood volume near the skin. That measurement depends on the conditions at the sensor. In its optical heart-rate guidance, Garmin specifically lists rowing among activities where wrist flexion can restrict local blood flow and affect tracking.
This gives rowers a plausible technical explanation for an incomplete recording. It does not establish the cause of every gap, or mean every watch will struggle on a rowing machine.
Start with the recording setup. Garmin recommends wearing the watch above the wrist bone, snug enough to stay in place but still comfortable. Check that the sensor is clean and select the activity profile that best matches the session. Its guidance notes that heart-rate calculations differ by activity type.
Pulling the strap as tight as possible is not the answer. Apple's measurement guidance also emphasises comfortable fit and close skin contact. It identifies movement and blood flow through the skin as factors that can affect the reading.
For the next row, I would make one setup change at a time and inspect the full graph afterwards. Look for whether the missing sections return. Treat this as troubleshooting the recording, rather than testing whether your fitness improved overnight.
If gaps persist and you use heart rate to pace sessions, a compatible chest strap is another recording option. Apple explicitly supports Bluetooth chest straps with Apple Watch; Garmin also recommends an external heart-rate strap when accuracy is critical. Check compatibility with the device or app that will record the workout before choosing one.
Keep the session's duration, machine distance and a short note about how it felt. Those details preserve useful information, but they cannot reconstruct the missing heart-rate samples. Avoid assigning the unrecorded minutes to a training zone just to complete the picture.
A rowing workout and its recording are two separate things to evaluate. First establish whether the sensor captured the session well enough. Then decide what the heart-rate data can tell you about the work.