The swimmers were outside of the park's sanctioned swimming area at about 7:30 p.m. when they began to struggle, according to Renton Fire Chief Mark Peterson.
Rescue swimmers from the Renton Fire Department reached the swimmers a short time later and brought them to shore.
One of the swimmers was taken to Harborview Medical Center in critical condition and the other was driven to a local hospital in stable condition, Peterson said.
A lifeguard was on duty at the park, but he didn't spot the swimmers because they were outside of the sanctioned area.
Peterson said the Rescue Swimmers program was implemented this year and was meant to save time in drowning situations. Unlike dive teams, the rescue swimmers don't have to stop to put on diving equipment.
Peterson didn't know the age or gender of the swimmers.