indoor rowing with runner 2

Your only choice is gym mode with an external HR strap. Gym mode as you are indoors and a chest strap as the internal HR does not work well for rowing. Low HR is often seen in rowing, cycling and weight lifting. Any time you do an activity that squeezes or tenses the forearms (like the pull stroke in rowing, bearing down on your handlebars in cycling, or virtually any weight lifting move) you are squeezing the blood vessels the watch is reading, so it sees this as a reduced pulse. It is not that the watch is having a problem reading your pulse; it is that your pulse at the wrist has dropped because you are temporarily cutting off blood flow to the vessels it is reading. I have experienced this with every optical HR I have used, including a Mio and a Scosche unit. For these sorts of activities, you are better off using a chest strap synced to the watch if getting a more exact reading is important to you.