"It was a really magical moment," Johnson tells PEOPLE exclusively of the engagement. "I was just so happy!"
Shark Tank's billionaire businessman Herjavec, 53, decorated with rose petals and placed the ring box in a bag atop an inflatable swan in the pool. When dance pro Johnson, 39, pulled in the swan, she opened the box and saw the 6.5 carat emerald cut ring.
"I was completely shocked," says Johnson with a laugh. "I can't stop staring at the ring. It's unbelievable."
For exclusive photos and much more from Robert and Kym, pick up the new issue of PEOPLE, on newsstands Friday
Following the proposal, Herjavec and Johnson, who began dating last spring, headed to his Los Angeles home friends and family gathered to celebrate.
"Robert flew my mom in from Australia," says Johnson. "It was very special."
Now, onto the wedding planning. Both Johnson and Herjavec says they will wed this year, and Herjavec in particular is looking forward to a certain coveted gift from the DWTS producers.
Says Herjavec: "Towards the end of the show, we realized that hopefully things were going to work out with [Kym and me]. The producers said if I ended up marrying her, they'd give me a mirrorball trophy!"