I'm venturing into uncharted territory. Actually the area is well-charted,
waterproof chart #52 San Francisco Bay. It's just that I'm planning on
using a marina that requires access via a narrow shallow channel.

It's the San Rafael Creek channel at approximately 37-57.8 N 122-27.5
W(Green 17) (I just eyeballed the position).

Outside the channel it is very shallow, 1 to 6 feet.

Along the channel it says (see note). Note: The controlling depth was 5.5
feet for a mid-width of 50 feet at channel entrance to San Rafael Creek
thence 2 feet for a width of 30 feet to the turning basin.

To me this means that the channel is 5.5 feet deep (plus or minus tide table
data). What does mid-width of 50 feet mean?

The 2 feet for a width of 30 feet confuses me. There's some pretty big
sailboats up the creek (so to speak) so I'm wondering how they manage to get
in and out given the average 4 to 8 foot draft of these boats.

Also in planning travel time, do such channels tend to have no wake
mandates; some of these channels are lengthy, for example the Petaluma River
entrance.

Thanks