Calories Burned Calculations

On average moderate rowing burns about 250 calories per 30 minutes of continuous exercise. On that basis you did potentially just over an hour of moderate rowing.
i guess the issue here is that from your description a lot of the time was spent not rowing and as such should have been stopped and started again when actually rowing

In my experience (based on a Fenix 3 watch), resting periods during a paddle do not add that much to the calorie count. I have lately done some very mixed paddling, some days as an instructor, paddling very slowly, and some days training rather hard on my own.

On slow days where my heart rate is below 90 BPM, I end up at around 2.25 kcal/minute. This seems pretty fixed so is probably some kind of calculation floor.

On days where I work harder, there is a quite linear relationship between BPM and kcal/minute. In my case it can be described as:
kcal/minute = 0.13 * (BPM - 78).

The OP burned 652 kcal in 150 minutes. That is 4.3 kcal/minute. To reach that, I would have to paddle with an average HR of 111 BPM which is above the OP's maximum heart rate for the entire trip.

Of course, these numbers are individual. The calculation constants in the watch are probably based on age, weight and sex (in my case 50 years, 83 kg and plenty). But it still seems odd that the OPP could reach such a high calorie count with so little effort.