They may have been Predaceous Diving Beetle adults. Here is one of genus Oreodytes from the S. Fork Snoqualmie:
It is only 2.2 mm, which is about as small of an adult as you will find, but some other genera have much larger adults. And yes, they swim incredibly fast with those two hind legs, which are lined with swimming hairs.

It is only 2.2 mm, which is about as small of an adult as you will find, but some other genera have much larger adults. And yes, they swim incredibly fast with those two hind legs, which are lined with swimming hairs.