McKenna and Bevilaqua were the first to exit the water in their respective fields in Geographe Bay after the 3.8km swim, with athletes now on the 180km bike course.
McKenna led a tight group of five professional male triathletes out of the water, with the South Australian just a second ahead of Lachlan Kerin, with Caleb Noble just behind in third.
Leigh Anderson-Voigt was fourth out of the water with Fraser Walsh rounding out the top five.
Perth-based athlete Kate Bevilaqua led the three female professionals out of the water, with Bevilaqua finishing the 3.8km swim in 59min 11seconds.
Bevilaqua returned to the beach more than nine minutes clear of Sarah Thomas, with 2021 IRONMAN Cairns champion Kylie Simpson a further 10 seconds behind in third.
GWM IRONMAN WESTERN AUSTRALIA SWIM LEG RESULTS
- Steve McKenna, AUS, 49.33
- Lachlan Kerin, AUS, 49.34
- Caleb Noble, AUS, 49.34
- Leigh Anderson-Voigt, AUS, 49.37
- Fraser Walsh, AUS, 49.40
- Tim Van Berkel, AUS, 51.29
- Blake Kappler, AUS, 51.29
- Matt Burton, AUS, 51.32
- Courtney Ogden, AUS, 51.40
- Simon Billeau, FRA, 58.16
- Kate Bevilaqua, AUS, 59.11
- Sarah Thomas, GBR, 1.08.22
- Kylie Simpson, AUS, 1.08.34