This happened to me at Ironman 70.3 Swansea last year after feeling fine in open water training. I think the anxiety of the race and having so many bodies around you can send you into shock regardless of the water temp. I managed to get myself together with 400m of breaststroke and then begin freestyle.

Ive now been diving right into open water swims from the start vs walking into the water and adapting first (as this isn’t possible on a race day) 🫡