I follow Angelo's argument.

When we create a prohibited line or area in the SI, we usually designate it as an obstruction, so that a tactically aggressive boat cannot 'herd' another boat across the line, causing them to break the rule.  If the line is an obstruction they cannot do this because RRS 19 requires them to give room.

Lets go back to the problem Jolene was trying to solve,

Boats, beating to the finishing line on starboard tack in the prevailing SW breeze at the venue, can hail for room to tack at Mark H if it is an obstruction, and will then tack onto port tack in the face of an oncoming fleet of starboard tack boats.  If outside boats within hail do not respond to the hail for room to tack, or if hailed boats do not pass on the hail there is a risk of collision.

I'll repeat the chart image, rotated to show top down, the prevailing SW breeze at the venue.

Note that the rocks or shallows referred to are the 2.6 and 2.9 spot depths towards the upper left of the image:  Mark H is not actually at or over the rocks and is surrounded by navigable water.

I think the Jolene and the club are trying to achieve a number of 'competing' aims:

1. Deter boats from sailing close to the shallows.
2. Remove the advantage that shallower draft boats may gain by sailing close to the shore or breakwater, where deeper draft boats cannot go.
3. Keep the sightline of the RO from the tower up the starting/finishing line unobstructed by boats not racing.
4. Not unnecessarily create a difficult RRS situation, with port tack boats crossing starboard tack boats approaching the finishing line.

• If the SI do not designate the prohibited line as an obstruction:
  • at the finishing line, RRS 18 Mark-Room applies at Mark H, but boats can pass on the wrong side of Mark H in safety and without breaking any other rule.
  • the risk of boats being forced to tack onto port in the face of oncoming starboard tack boats is avoided.
  • there is a risk that tactically aggressive boats can 'herd' other boats across the prohibited line, causing them to break a rule and be penalised.
• If the SI designate the prohibited line as an obstruction:
  • at the finishing line, RRS 19 and 20 apply at Mark H, and a boat may hail for and be entitled to room to tack onto port across the finishing fleet on starboard
  • the risk of boats being 'herded' across the prohibited line and penalised is avoided.

There may be several reasonable opportunities before finishing when boats cross each other on different legs.