There was a tsunami warning for Crescent City and points nearby one night when my daughter and I were out on the Lost Coast late May 2008. We didn't know anything about it, but it worried my wife who got on the phone with local sheriff's office who seemed aware of the problem, though she had the impression it wasn't the highest thing on their priority list. I doubt that it was a problem. There is no channel there to funnel the water or increase its speed. If the tsunami height was as reported, and it was low tide, I doubt that backpackers there would particularly have noticed much unless they happened to be right down near the water. Higher wave and a high tide, though, could be an issue.
Link to rough map of our trip