You're absolutely right, the latest chart does show 2' in March 2015. See below.
Good thing I checked my logbook. I was actually in the Knapps Narrows on September 28, 2014 not 2015. We stayed overnight at the very nice Knapps Narrows Marina and Inn. There were several other sailboats along their floating dock
where there was plenty of water
I remember it was tricky getting in, but we found deep enough water along one edge of the channel. Going out, no problem as we now knew the channel.
Still, that's a lot of shoaling in a short time. Sure hope the COE can get it together and get it dredged. It's a very nice stop.