Heartbeat unbelievable high using the Garmin heartbeat band with Forerunners 245 while rowing.

By Mac you mean the max recorded during the activity or the max hr in the user settings detected and set by the watch?

Without max I mean the highest heart rate according to the Garmin heart band and Garmin 245  watch. While I have the rowing app Outdoor rowing running on my Watch.

What reported the 190 bpm max? OHR or external HRM band?

I was wearing the heart band and the watch. I use my bike to get to the boats and the heart band is from tat moment connected to my watch.

When you look at the recorded activity in Garmin Connect, go to the Charts, do you see a graph called Respiratory Rate? If yes, does it have any gaps in it?

You only get Respiratory Rate when the HR is reported from the band, so if there are gaps in it it means that for that period of time the band disconnected and you see the HR from the watch

I always go by bike to the rowing club and use the band from that moment. When I activate my watch for cycling, I get a message that the band is connected. I see no Respiratory Rate in both cycling and rowing Charts. I see under training effect Aeroob and Anaeraoob. Respiration Rate is not available.

You get the training effect even without the band, that doesn't mean anything. However if you would get the HR from the band then you would also see the respiratory rate as far as I know. That is the way I always check if it disconnected or not. So from what you say it sounds like you don't get the HR from the band but from the watch.