Fenix 5x Plus open water accuracy

I’ve been using my brand new Fenix 5x Plus watch while on holiday. My main sport is running but I have competed in Ironman and SwimRun races and need a watch that can deal with both running and swimming effectively. I previously used a Fenix 3 very happily. The 5x plus is disastrous when it comes to open water swimming. No matter whether I use GPS on its own or with GLONASS or Galileo, the track bears no resemblance to the actual swim. Generally it records less than half of the distance. I have tried waiting for five minutes before starting to swim but that makes no difference either. I swim a normal front crawl stroke and I’m swimming in clear salt water with gorgeous blue skies above me. Considering I’m using Garmin’s most expensive watch, I’m very disappointed. Support suggests I attach the watch to a float or put it under my swim cap, both of which are either laughable or offensive depending on my mood. Is anybody else having similar issues?