@joex – I remember seeing your previous post and took some photos for you.
No HR data, but this is my low tech solution, Argos watch & Suba diving pad from Ebay.
Ignore the terrible writing & shorthand, but I know my last morning’s swim added up to 3,000m.
Note - My eyesight is so bad I can’t see the clock, so use the watch for all going times. 
Cheers, Paul. 