A lake doesnt have to be marked by stakes to be a penalty area, it is by definition, obviously a lake is a lake. The only purpose of the stakes is to define it as Red or Yellow. This has always been the case for the old rules as well, the natural border of the lake/creek is in fact the margin of the penalty area, unless otherwise marked.
A penalty area is:

  • Any body of water on the course (whether or not marked by the Committee), including a sea, lake, pond, river, ditch, surface drainage ditch or other open watercourse (even if not containing water), and
  • Any other part of the course the Committee defines as a penalty area.
It comes down to that definition whether its a penalty area or the general area.

IMO for example an unstaked shallow depression that runs across the fairway to channel water that is cut to fairway height is definitely not a "drainage ditch", even if at some point there is a pipe somewhere along it.