Piha competitor Emily Pearce, who was in the middle of the field, quickly changed course and swam to Ms Ryan, abandoning any chance of winning or placing in the race.
"I was just swimming and noticed one of the girls had stopped," Ms Pearce said. "I waited with her and said 'are you okay?' and she didn't answer."
Ms Pearce, a 19-year-old Auckland University architecture student, held Ms Ryan and kept her afloat for about 30 seconds until rescuers arrived.
"To be honest, I didn't know exactly what was wrong with her. She stopped and it just seemed like the right thing to do [to help]."
Thankfully for both of them, it didn't take too long for a rescue boat to arrive.
Ms Ryan was taken to shore and "sorted out pretty quick" by medics, while Ms Pearce kept swimming, finishing near the back of the field.
Ms Ryan was grateful for Ms Pearce's actions.
"It was pretty bloody amazing, you don't really expect anyone to stop. It was just a real nice gesture," Ms Ryan said.
Ms Pearce, who is a former top age-group triathlete, capped off a memorable day by winning the open women's 2km beach run.