Watch that measures heart rate underwater as accurate as possible

I've been doing some research and it seems like every watch sucks at reading your heart rate while your swimming (pool). I don't want to weat a chest strap and anylize an Excel file on my laptop. I want a watch, wear at during my swimming sessions and then sync it with my phone.

I really don't care about special features, style recognition, specific workouts than can be chosen, vibration or anything else.. I just want accurate heart rate data. What would you recommend?