Nick,
I just did a google search for "otters in Yosemite" and got your post on this forum which I just joined.
Anyway, a friend and I just completed a trek that included a cross country leg from Boundary Lake to Flora Lake. The route followed Bartlet Creek which has several unnamed lakes on its course to Flora. We stopped for lunch at one of the middle lakes (I'm not sure which one) and there was definitly an otter there. It was the first either of us had seen in the park. Neither of us even knew otters were in Yosemite.
This otter was not the big fat looking ones you see on nature programs but seemed to be a little leaner. It had a white area under the chin down onto the chest and a tapered tail. We both agreed that it was an otter. We got a few pictures but the thing was just far enough away that the image isn't that good. It did some swimming and diving and spent some time sunning itself on a rock in the lake.
There must be quite a few otters in the park.
Old Dude