That is a big ship marker marking Matinecook Pt and going outside it is good seamanship. A court of inquiry would define going inside negligence if a serious accident occurred.
Cutting corners at night without local knowledge sinks boats!
It is also bad seamanship to assume that charts and chart plotters are 100% accurate. The mark 1 eyeball is much more reliable.
The rocky bottom was deep but if those soundings are in feet and you had crossed that 10ft mark in any kind of waves you might have bounced the keel through the hull.