I saw a lot of people wanting to record hr during lap swimming in the other thread. Not sure whether anyone has mentioned this hack before, there is indeed a way to do this. I have used this hack for over a year, it works fine for me.
Here is how I do it in my Instinct.
First, you need to assign a "hotkey" to broadcast hr. I am using the long-press BACK key as my broadcasting key.
Then, when you select "pool swim" on your watch, before you start the activity (i.e. when the OPTIONS flashing on your screen), long-press the broadcasting key (in my case, it is the BACK key), this will take you to the broadcasting heart rate screen. Start broadcasting hr, then press BACK key. This will take you back to the previous pool swim screen. Now if you start activity, the heart rate recording will continue and will not be turned off. If you check the backlight on the watch, you will see the green light is on.
Hopefully, this will help. I am not sure how accurate the hr in Instinct watch will be in the water. Mine seems reasonable, it goes up when I work hard, it goes down when I take a break 
(this hack works for open water swim as well).
