i think that given the completeness of under sea surveying at the time, nautical charts only showed the approximate depth of coastal waters in many areas.
Nautical charts which commanders would use for the purpose of navigation were all white as far as im aware, there were now shaded blue spots to show depth... only a series of isogonic lines which showed the approximate depth.
a skipper might try to exploit one of these "holes" but there is no guarantee that such a hole was charted at all... and even if it was there was no guarantee as to its depth.
given the accuracy of available information back then, exploiting one of these holes would probably be considered a risk... the worthiness of the risk - up to the individual commander.
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