Looking for Swimming Heart Rate Monitor

I had a mini stroke April 2017, and am doing cardio and strength exercises in the pool. I have a Fitbit Charge HR but cannot take it in the water. I would like to know if anyone has some sort of tracker they use in the water that they can look and see their heart rate while working out. I am trying to make sure I hit my max exercise heart rate, and have been doing to cardio portions using the HITT method. but want to make sure I am getting optimum exposure. Any tips would be great appreciated. Thanks in Advance.

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