Ultrawatch 2 and heartrate

Have you tried to restart the watch to see if the issues persist?

If the issues persist, are they only happening on open water swims or also on other workouts? If it is occurring on other workouts as well you might need to set up hardware service for your watch

If it Is only at the open water swim, you might want to give this as feedback to apple so that they can look in to it, do so using this link

Product Feedback - Apple

I guess, you are wearing the Ocean Band?

During the workouts with my Ultra 2, I'm wearing the watch a bit more further away from the wrist, to get a solid heart rate detection.

Water temperature could also be a factor. I didn't test it below 17C (62F) this summer, but the watch registered the heart rate every time I started the Open Water Swim workout.

Just like running outdoors in Winter, the blood vessels may be harder to detect.