What is this bug that keeps appearing, just one or two at a time, in our sink? We also occasionally find one in our cats’ water.

They aren’t being pests, but we have to keep gently moving them out of the bathroom sink before we can use it, for fear of drowning them!

Any idea what this is?:

The first in an unevenly-spaced series of pictures for which we ask you, “What is this?”