> I'm also surprised there's no contour lines between the depths (although perhaps this is done to not infer a knowledge of depths in the intermediate points where perhaps there is no such knowledge).

Yes, contour lines would imply accuracy that isn't there. The depth numbers indicate positions where the depth was actually measured. Any other location that doesn't have a number was not measured. Bear in mind that even if you have a depth number on your chart, it might be old and out of date. Safe navigation is never guaranteed. And subs never run depth-finder sonar when on patrol.