I arrived in the same area a few days ago. Heading across the Okee after I fix my busted
alternator.
I think, like you said, the charted depths near on each side of the bridge is a good indication of the depths through the side spans.
Also noticing where the local
power boats use the side spans can help here. Even with thier
draft, which often could be near yours, I feel they would prefer to have a comfortable depth under
hull.
Lots of shallows in wide open waters around here. And the traffic is crazy on the weekends.
I did notice several local sailors using
parts of the area outside of the causeway bridge to Sanibel that I wouldn't venture into when I arrived from
offshore coming around from south of the island. So it's a matter of gaining local knowledge.
I've been accused of using the Braille method for acquiring this back in
Alabama. :-) And I have redundant depth sounders.