Incorrect HR detected while swim session

Incorrect HR detected while swim session. While, I swim daily for 6 days a week - and watch works fine but once or twice in a month I do notice that if I am swimming continuously (where my body remains horizontal), the HR tracked is not right. Today as part of the session which lasted 1 hour with no breaks in freestyle form, where I covered 2575 mts. it recorded an avg. 101 bpm which is normally 150s and thus instead of around 120 intensity mins, it just captured 62. 

Take this is a bug and work on it

And help me to update my intensity mins manually (since I know very well how much it would be)