@DavidCuenca
Hey,
About the GPS accuracy, waiting on shore is indeed a good idea, but you should try staying longer (12 minutes and a half precisely).
Basically, all satellites broadcast the positions of the other sats in their constellation, as well as atmospheric conditions, this data is the mos up to date (updated every 2 hours, valid 4 hours) but the bit rate is very slow (Galileo has a higher bit rate and needs only about 6 minutes) :
https://en.wikipedia.org/wiki/GPS_signals#Almanac
That should be more efficient than lifting your arms ^^
About HR, the HRM of 9PP isn’t great, but what worked for me is to wear the watch a lot higher on the wrist (almost 1 third of the arms length), and have it almost uncomfortably tight