The Petaluma needs a dredge, no question. Make your trip near the end of a flood and stay in the middle of the channel. The Petaluma is NOT a river. It's a tidal slough. The "river" designation was given years ago when the local politicians were after federal money for its dredging. Therefore, the "outside of the bends are deepest", that we see in rivers, aren't true for the Petaluma slough.
We draw about 4' x 2" and have never had a problem when transiting in high water. When you reach the turning basin, stay close to the wharf on the south side of the basin. It's shallow everywhere else!
The problem with going up the Napa River is it takes most of the day, but the city doesn't allow overnight stays at their dock. If you do choose the Napa, which, by the way, is a beautiful trip, pick your times for an incoming flood.
1986 GB-42 Classic