does the apple watch heart rate sensor works while you are swimming?

The watch with Watch OS 3 can.

see the keynote, they discuss at length they were able to do this.

Hi

Apple has not yet stated whether or not, for Apple Watch Series 2 models, the Workout app monitors heart rate during swimming workouts.

What Apple has stated is that the watch can count laps, track average lap pace and automatically detect your stroke type, which enables it to accurately measure your Active Calories burned during a swimming workout.