I sailed with my chest sensor HR monitor and watch that I usually only use for running the other day. I too was interested to see what HR max and variability there would be. B&J session, with one stop back on the beach.
I barely looked at the readout during the session, but the occasional glance did confirm how I was feeling at the time- cruisy- HR in the moderate range (128-145bpm), shagged- HR in the maximal effort range (165-183,but never at HR max of 183). Interesting to see your performance after the event.
I have found that when I'm in the waves I have to focus on relaxing and getting my HR and breathing rate down to conserve energy, and to get a longer session. So a bit like yoga on the water, heading out the back and recovering. Haven't used the monitor in waves yet, but I'd like to see some confirmation that the reason I'm shagged is because my HR has been up there in the extreme range, and that I can consciously recover asap.