Submitter does not have a specimen
I spotted the animal while it was swimming away from me. I first thought it was an otter or beaver but then it dove and I could see a long skinny rat-like tail. I did not know what it was but a friend suggested a nutria, and after looking at pictures of them online, I've concluded that a nutria is what it likely was. I've walked through the park from Knapp street to the Bybee/Hwy 99 ramp intersection most weekday mornings for four years and have never seen any mammals such as this.