Urban Legend, Folklore, And Tall Tale

Omak lake is rumored to have some type of "sea-monster". One spring a few years back Enlightened and I had hike way down a gravel beach looking for cruisers. I was up on higher ground spotting for enlightened, who was a little over knee deep down below me. I spotted something big about four or five feet in front of her. At first I thought it was the mother of all cutthroat, but I realized it was using four flippers to propel itself through the water, not a tail.
I watched and waited, hoping an otter would pop up for air to explain it. I've never seen an otter in that saline lake before or since, but it seemed to be about the right size (sixty or seventy pounds) to be a large otter. I could see the calm surface of the lake for about two miles in either direction. Nothing ever surfaced for air.
"Did you see that?" I finally asked. "No. What, was it a big one?"
"Yeah, but it wasn't a fish." I say.
"What! Why didn't you say something so I could get out of the water?
"I didn't want you to spook it, or worse, trigger an aggressive reaction. I don't think it even saw you standing still like that." I answered.
"Oh."

To this day I still have no idea what I saw.